diff --git a/akka-stream/src/main/scala/akka/stream/impl/ActorFlowMaterializerImpl.scala b/akka-stream/src/main/scala/akka/stream/impl/ActorFlowMaterializerImpl.scala
index 9d6b9cdc5b..a6a4f91c59 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/ActorFlowMaterializerImpl.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/ActorFlowMaterializerImpl.scala
@@ -118,7 +118,7 @@ private[akka] case class ActorFlowMaterializerImpl(override val settings: ActorF
                 (UnfairMerge.props(effectiveSettings, shape.inlets.size), shape.preferred +: shape.inArray.toSeq, shape.out)
 
               case ConcatModule(shape, _) ⇒
-                require(shape.inArray.size == 2, "currently only supporting concatenation of exactly two inputs") // FIXME
+                require(shape.inArray.size == 2, "currently only supporting concatenation of exactly two inputs") // TODO
                 (Concat.props(effectiveSettings), shape.inArray.toSeq, shape.out)
 
               case zip: ZipWithModule ⇒
@@ -270,7 +270,7 @@ private[akka] object ActorProcessorFactory {
       case GroupBy(f, _)              ⇒ (GroupByProcessorImpl.props(settings, f), ())
       case PrefixAndTail(n, _)        ⇒ (PrefixAndTailImpl.props(settings, n), ())
       case SplitWhen(p, _)            ⇒ (SplitWhenProcessorImpl.props(settings, p), ())
-      case ConcatAll(_)               ⇒ (ConcatAllImpl.props(materializer), ()) //FIXME closes over the materializer, is this good?
+      case ConcatAll(_)               ⇒ (ConcatAllImpl.props(materializer), ())
       case StageFactory(mkStage, _)   ⇒ (ActorInterpreter.props(settings, List(mkStage()), materializer), ())
       case TimerTransform(mkStage, _) ⇒ (TimerTransformerProcessorsImpl.props(settings, mkStage()), ())
       case MaterializingStageFactory(mkStageAndMat, _) ⇒
diff --git a/akka-stream/src/main/scala/akka/stream/impl/ActorProcessor.scala b/akka-stream/src/main/scala/akka/stream/impl/ActorProcessor.scala
index b8f5c29f2e..2de23fb189 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/ActorProcessor.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/ActorProcessor.scala
@@ -249,7 +249,6 @@ private[akka] abstract class ActorProcessorImpl(val settings: ActorFlowMateriali
   with ActorLogging
   with Pump {
 
-  // FIXME: make pump a member
   protected val primaryInputs: Inputs = new BatchingInputBuffer(settings.initialInputBufferSize, this) {
     override def inputOnError(e: Throwable): Unit = ActorProcessorImpl.this.onError(e)
   }
diff --git a/akka-stream/src/main/scala/akka/stream/impl/FanoutProcessor.scala b/akka-stream/src/main/scala/akka/stream/impl/FanoutProcessor.scala
index ed37d664bd..fb3ac2c8a2 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/FanoutProcessor.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/FanoutProcessor.scala
@@ -79,11 +79,9 @@ private[akka] abstract class FanoutOutputs(val maxBufferSize: Int, val initialBu
     case SubscribePending ⇒
       subscribePending()
     case RequestMore(subscription, elements) ⇒
-      // FIXME can we avoid this cast?
       moreRequested(subscription.asInstanceOf[ActorSubscriptionWithCursor[Any]], elements)
       pump.pump()
     case Cancel(subscription) ⇒
-      // FIXME can we avoid this cast?
       unregisterSubscription(subscription.asInstanceOf[ActorSubscriptionWithCursor[Any]])
       pump.pump()
   }
diff --git a/akka-stream/src/main/scala/akka/stream/impl/FuturePublisher.scala b/akka-stream/src/main/scala/akka/stream/impl/FuturePublisher.scala
index 5fd0d05ab3..7b6141e25f 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/FuturePublisher.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/FuturePublisher.scala
@@ -142,7 +142,7 @@ private[akka] class FuturePublisher(future: Future[Any], settings: ActorFlowMate
     }
   }
 
-  override def postStop(): Unit = // FIXME if something blows up, are the subscribers onErrored?
+  override def postStop(): Unit =
     if (exposedPublisher ne null)
       exposedPublisher.shutdown(shutdownReason)
 
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala
index b74022efb0..1bdbcd25de 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala
@@ -173,7 +173,7 @@ private[akka] final case class Grouped[T](n: Int) extends PushPullStage[T, immut
   override def onPull(ctx: Context[immutable.Seq[T]]): SyncDirective =
     if (ctx.isFinishing) {
       val elem = buf.result()
-      buf.clear() //FIXME null out the reference to the `buf`?
+      buf.clear()
       left = n
       ctx.pushAndFinish(elem)
     } else ctx.pull()
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
index 713b892ab4..506dc7f42c 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
@@ -223,7 +223,7 @@ class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends Graph
    * `n` must be positive, otherwise IllegalArgumentException is thrown.
    */
   def grouped(n: Int): javadsl.Flow[In, java.util.List[Out @uncheckedVariance], Mat] =
-    new Flow(delegate.grouped(n).map(_.asJava)) // FIXME optimize to one step
+    new Flow(delegate.grouped(n).map(_.asJava)) // TODO optimize to one step
 
   /**
    * Similar to `fold` but is not a terminal operation,
@@ -249,7 +249,7 @@ class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends Graph
    * IllegalArgumentException is thrown.
    */
   def groupedWithin(n: Int, d: FiniteDuration): javadsl.Flow[In, java.util.List[Out @uncheckedVariance], Mat] =
-    new Flow(delegate.groupedWithin(n, d).map(_.asJava)) // FIXME optimize to one step
+    new Flow(delegate.groupedWithin(n, d).map(_.asJava)) // TODO optimize to one step
 
   /**
    * Discard the given number of elements at the beginning of the stream.
@@ -371,7 +371,7 @@ class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends Graph
    * the element is dropped and the stream and substreams continue.
    */
   def groupBy[K](f: japi.Function[Out, K]): javadsl.Flow[In, akka.japi.Pair[K, javadsl.Source[Out @uncheckedVariance, Unit]], Mat] =
-    new Flow(delegate.groupBy(f.apply).map { case (k, p) ⇒ akka.japi.Pair(k, p.asJava) }) // FIXME optimize to one step
+    new Flow(delegate.groupBy(f.apply).map { case (k, p) ⇒ akka.japi.Pair(k, p.asJava) }) // TODO optimize to one step
 
   /**
    * This operation applies the given predicate to all incoming elements and
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
index 1f6f1eb9dd..bdfef7f87b 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
@@ -3,6 +3,7 @@
  */
 package akka.stream.javadsl
 
+import scala.collection.immutable
 import java.util.concurrent.Callable
 import akka.actor.{ Cancellable, ActorRef, Props }
 import akka.japi.Util
@@ -95,9 +96,21 @@ object Source {
    * Iterator, but every Subscriber directly attached to the Publisher of this
    * stream will see an individual flow of elements (always starting from the
    * beginning) regardless of when they subscribed.
+   *
+   * Make sure that the `Iterable` is immutable or at least not modified after
+   * being used as a `Source`. Otherwise the stream may fail with
+   * `ConcurrentModificationException` or other more subtle errors may occur.
    */
-  def from[O](iterable: java.lang.Iterable[O]): javadsl.Source[O, Unit] =
-    new Source(scaladsl.Source(akka.stream.javadsl.japi.Util.immutableIterable(iterable)))
+  def from[O](iterable: java.lang.Iterable[O]): javadsl.Source[O, Unit] = {
+    // this adapter is not immutable if the the underlying java.lang.Iterable is modified
+    // but there is not anything we can do to prevent that from happening.
+    // ConcurrentModificationException will be thrown in some cases.
+    val scalaIterable = new immutable.Iterable[O] {
+      import collection.JavaConverters._
+      override def iterator: Iterator[O] = iterable.iterator().asScala
+    }
+    new Source(scaladsl.Source(scalaIterable))
+  }
 
   /**
    * Start a new `Source` from the given `Future`. The stream will consist of
@@ -366,7 +379,7 @@ class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Graph[Sour
    * @param n must be positive, and `d` must be greater than 0 seconds, otherwise [[IllegalArgumentException]] is thrown.
    */
   def groupedWithin(n: Int, d: FiniteDuration): javadsl.Source[java.util.List[Out @uncheckedVariance], Mat] =
-    new Source(delegate.groupedWithin(n, d).map(_.asJava)) // FIXME optimize to one step
+    new Source(delegate.groupedWithin(n, d).map(_.asJava)) // TODO optimize to one step
 
   /**
    * Discard the given number of elements at the beginning of the stream.
@@ -476,7 +489,7 @@ class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Graph[Sour
    * to consume only one of them.
    */
   def groupBy[K](f: japi.Function[Out, K]): javadsl.Source[akka.japi.Pair[K, javadsl.Source[Out @uncheckedVariance, Unit]], Mat] =
-    new Source(delegate.groupBy(f.apply).map { case (k, p) ⇒ akka.japi.Pair(k, p.asJava) }) // FIXME optimize to one step
+    new Source(delegate.groupBy(f.apply).map { case (k, p) ⇒ akka.japi.Pair(k, p.asJava) }) // TODO optimize to one step
 
   /**
    * This operation applies the given predicate to all incoming elements and
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/japi/Util.scala b/akka-stream/src/main/scala/akka/stream/javadsl/japi/Util.scala
deleted file mode 100644
index 2024476d39..0000000000
--- a/akka-stream/src/main/scala/akka/stream/javadsl/japi/Util.scala
+++ /dev/null
@@ -1,17 +0,0 @@
-/**
- * Copyright (C) 2014 Typesafe Inc. <http://www.typesafe.com>
- */
-package akka.stream.javadsl.japi
-
-import scala.collection.immutable
-
-object Util {
-
-  import collection.JavaConverters._
-  // FIXME this does not make something an immutable iterable!!
-  def immutableIterable[T](iterable: java.lang.Iterable[T]): immutable.Iterable[T] =
-    new immutable.Iterable[T] {
-      override def iterator: Iterator[T] = iterable.iterator().asScala
-    }
-
-}
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/japi/WithVariance.scala b/akka-stream/src/main/scala/akka/stream/javadsl/japi/WithVariance.scala
index 522ddd2bb4..5b31a67ac1 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/japi/WithVariance.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/japi/WithVariance.scala
@@ -9,7 +9,7 @@ package akka.stream.javadsl.japi
 /**
  * A Function interface. Used to create first-class-functions is Java.
  */
-@SerialVersionUID(1L) // FIXME: add variance to akka.japi and remove this akka.stream.japi!
+@SerialVersionUID(1L) // TODO: add variance to akka.japi and remove this akka.stream.japi!
 trait Function[-T, +R] {
   @throws(classOf[Exception])
   def apply(param: T): R
@@ -18,7 +18,7 @@ trait Function[-T, +R] {
 /**
  * A Function interface. Used to create 2-arg first-class-functions is Java.
  */
-@SerialVersionUID(1L) // FIXME: add variance to akka.japi and remove this akka.stream.japi!
+@SerialVersionUID(1L) // TODO: add variance to akka.japi and remove this akka.stream.japi!
 trait Function2[-T1, -T2, +R] {
   @throws(classOf[Exception])
   def apply(arg1: T1, arg2: T2): R
@@ -27,7 +27,7 @@ trait Function2[-T1, -T2, +R] {
 /**
  * A constructor/factory, takes no parameters but creates a new value of type T every call.
  */
-@SerialVersionUID(1L) // FIXME: add variance to akka.japi and remove this akka.stream.japi!
+@SerialVersionUID(1L) // TODO: add variance to akka.japi and remove this akka.stream.japi!
 trait Creator[+T] extends Serializable {
   /**
    * This method must return a different instance upon every call.
@@ -39,7 +39,7 @@ trait Creator[+T] extends Serializable {
 /**
  * A Procedure is like a Function, but it doesn't produce a return value.
  */
-@SerialVersionUID(1L) // FIXME: add variance to akka.japi and remove this akka.stream.japi!
+@SerialVersionUID(1L) // TODO: add variance to akka.japi and remove this akka.stream.japi!
 trait Procedure[-T] {
   @throws(classOf[Exception])
   def apply(param: T): Unit
@@ -48,7 +48,7 @@ trait Procedure[-T] {
 /**
  * Java API: Defines a criteria and determines whether the parameter meets this criteria.
  */
-@SerialVersionUID(1L) // FIXME: add variance to akka.japi and remove this akka.stream.japi!
+@SerialVersionUID(1L) // TODO: add variance to akka.japi and remove this akka.stream.japi!
 trait Predicate[-T] {
   def test(param: T): Boolean
 }
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/FlexiRoute.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/FlexiRoute.scala
index e16ac52926..b982fba64e 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/FlexiRoute.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/FlexiRoute.scala
@@ -240,7 +240,6 @@ abstract class FlexiRoute[In, S <: Shape](val shape: S, attributes: OperationAtt
     case None    ⇒ super.toString
   }
 
-  // FIXME what to do about this?
   override def withAttributes(attr: OperationAttributes): Graph[S, Unit] =
     throw new UnsupportedOperationException(
       "withAttributes not supported by default by FlexiRoute, subclass may override and implement it")