diff --git a/akka-http-core/src/main/scala/akka/http/engine/server/HttpServer.scala b/akka-http-core/src/main/scala/akka/http/engine/server/HttpServer.scala
index ddb57ee22a..6f1a0d6288 100644
--- a/akka-http-core/src/main/scala/akka/http/engine/server/HttpServer.scala
+++ b/akka-http-core/src/main/scala/akka/http/engine/server/HttpServer.scala
@@ -67,6 +67,14 @@ private[http] object HttpServer {
             HttpRequest(effectiveMethod, effectiveUri, headers, createEntity(entityParts), protocol)
         }
 
+    // we need to make sure that only one element per incoming request is queueing up in front of
+    // the bypassMerge.bypassInput. Otherwise the rising backpressure against the bypassFanout
+    // would eventually prevent us from reading the remaining request chunks from the transportIn
+    val bypass = Flow[RequestOutput].filter {
+      case (_: RequestStart | _: MessageStartError) ⇒ true
+      case _                                        ⇒ false
+    }
+
     val rendererPipeline =
       Flow[ResponseRenderingContext]
         .section(name("recover"))(_.transform(() ⇒ new ErrorsTo500ResponseRecovery(log))) // FIXME: simplify after #16394 is closed
@@ -82,7 +90,7 @@ private[http] object HttpServer {
     Flow() { implicit b ⇒
       //FIXME: the graph is unnecessary after fixing #15957
       transportIn ~> requestParsing ~> bypassFanout ~> requestPreparation ~> serverFlow ~> bypassMerge.applicationInput ~> rendererPipeline ~> transportOut
-      bypassFanout ~> bypassMerge.bypassInput
+      bypassFanout ~> bypass ~> bypassMerge.bypassInput
       oneHundredContinueSource ~> bypassMerge.oneHundredContinueInput
 
       b.allowCycles()
@@ -107,7 +115,7 @@ private[http] object HttpServer {
       override val initialState = State[Any](Read(bypassInput)) {
         case (ctx, _, requestStart: RequestStart)      ⇒ waitingForApplicationResponse(requestStart)
         case (ctx, _, MessageStartError(status, info)) ⇒ finishWithError(ctx, "request", status, info)
-        case _                                         ⇒ SameState // drop other parser output
+        case _                                         ⇒ throw new IllegalStateException
       }
 
       def waitingForApplicationResponse(requestStart: RequestStart): State[Any] =