I was making an uber-build, and got this in the build output while compiling Scala IDE. Then it hung.

2013-02-28 09:50:20,765  INFO [main] - ScalaSourceFile - [debug] buildStructure for L/debug/src/stepping/MethodClassifiers.scala (/home/luc/dev/scala-ide/uber-build/scala-ide/org.scala-ide.sdt.debug.tests/target/work/data/debug/src/stepping/MethodClassifiers.scala)
2013-02-28 09:50:20,832  INFO [main] - ScalaSourceFile - [debug] buildStructure for L/debug/src/stepping/ForComprehensionListIntOptimized.scala (/home/luc/dev/scala-ide/uber-build/scala-ide/org.scala-ide.sdt.debug.tests/target/work/data/debug/src/stepping/ForComprehensionListIntOptimized.scala)
2013-02-28 09:50:21,114  INFO [Worker-2] - ScalaDebugTestSession - SUSPENDED at: ForComprehensionListIntOptimized$.main(Array[String]):11
2013-02-28 09:50:21,136  INFO [Worker-1] - ScalaDebugTestSession - SUSPENDED at: ForComprehensionListIntOptimized$$anonfun$main$1.apply$mcVI$sp(Int):12
2013-02-28 09:50:21,426  INFO [Worker-2] - ScalaDebugTestSession - SUSPENDED at: ForComprehensionListIntOptimized$.foo(List):21
2013-02-28 09:50:21,452  INFO [Worker-4] - ScalaDebugTestSession - SUSPENDED at: ForComprehensionListIntOptimized$$anonfun$foo$1.apply$mcVI$sp(Int):22
2013-02-28 09:50:21,795  INFO [Worker-4] - ScalaDebugTestSession - SUSPENDED at: ForComprehensionListIntOptimized.this(List):31
2013-02-28 09:50:21,830  INFO [Worker-3] - ScalaDebugTestSession - SUSPENDED at: ForComprehensionListIntOptimized$$anonfun$1.apply$mcVI$sp(Int):32
2013-02-28 09:50:22,156  INFO [Worker-4] - ScalaDebugTestSession - SUSPENDED at: ForComprehensionListIntOptimized.bar():37
2013-02-28 09:50:22,194  INFO [Worker-1] - ScalaDebugTestSession - SUSPENDED at: ForComprehensionListIntOptimized$$anonfun|$1.apply$mcVI$sp(Int):38
2013-02-28 09:50:22,491  INFO [Worker-4] - ScalaDebugTestSession - SUSPENDED at: AnonFunOnListString$.a(List):19
2013-02-28 09:50:22,505 ERROR [ForkJoinPool-6-worker-4] - org.scala-ide.sdt.core - org.scala-ide.sdt.core - org.scala-ide.sdt.core - 0 - Shutting down scala.tools.eclipse.debug.breakpoints.BreakpointSupportActor@257cae95 because of
org.eclipse.core.internal.resources.ResourceException: Marker id 21 not found.
	at org.eclipse.core.internal.resources.Marker.checkInfo(Marker.java:57)
	at org.eclipse.core.internal.resources.Marker.setAttribute(Marker.java:237)
	at org.eclipse.core.internal.resources.Marker.setAttribute(Marker.java:259)
	at scala.tools.eclipse.debug.breakpoints.RichBreakpoint.setVmRequestEnabled(RichBreakpoint.scala:28)
	at scala.tools.eclipse.debug.breakpoints.BreakpointSupportActor.scala$tools$eclipse$debug$breakpoints$BreakpointSupportActor$$changed(BreakpointSupport.scala:150)
	at scala.tools.eclipse.debug.breakpoints.BreakpointSupportActor$$anonfun$behavior$1.applyOrElse(BreakpointSupport.scala:114)
	at scala.PartialFunction$OrElse.apply(PartialFunction.scala:162)
	at scala.actors.ReactorTask.run(ReactorTask.scala:31)
	at scala.actors.Reactor$class.resumeReceiver(Reactor.scala:129)
	at scala.tools.eclipse.debug.breakpoints.BreakpointSupportActor.scala$actors$InternalReplyReactor$$super$resumeReceiver(BreakpointSupport.scala:95)
	at scala.actors.InternalReplyReactor$class.resumeReceiver(InternalReplyReactor.scala:59)
	at scala.tools.eclipse.debug.breakpoints.BreakpointSupportActor.resumeReceiver(BreakpointSupport.scala:95)
	at scala.actors.InternalActor$class.searchMailbox(InternalActor.scala:76)
	at scala.tools.eclipse.debug.breakpoints.BreakpointSupportActor.searchMailbox(BreakpointSupport.scala:95)
	at scala.actors.Reactor$$anonfun$startSearch$1$$anonfun$apply$mcV$sp$1.apply$mcV$sp(Reactor.scala:117)
	at scala.actors.Reactor$$anonfun$startSearch$1$$anonfun$apply$mcV$sp$1.apply(Reactor.scala:114)
	at scala.actors.Reactor$$anonfun$startSearch$1$$anonfun$apply$mcV$sp$1.apply(Reactor.scala:114)
	at scala.actors.ReactorTask.run(ReactorTask.scala:33)
	at scala.actors.ReactorTask.compute(ReactorTask.scala:63)
	at scala.concurrent.forkjoin.RecursiveAction.exec(RecursiveAction.java:160)
	at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:262)
	at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:975)
	at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1478)
	at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:104)

I added a bit of context, it might help to find which test failed. It was in debug.tests.