This happens randomly. When a constraint breaks as it hits something physically while running in PIE. It hitches the entire application for 5-50 seconds, then resumes, outputting this in the log:
2016.01.20-22.25.53 [REDCARBON-5236] 01592.283: LogOutputDevice: === Handled error: ===
Ensure condition failed: GetShadowIndex() == 0 [File:D:\BuildFarm\buildmachine_++depot+UE4-Releases+4.10\Engine\Source\Runtime\Core\Public\HAL\IConsoleManager.h] [Line: 761]
Stack:
UE4Editor-Core.dll!FWindowsPlatformStackWalk::StackWalkAndDump() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\private\windows\windowsplatformstackwalk.cpp:175]
UE4Editor-Core.dll!FDebug::EnsureFailed() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\private\misc\outputdevice.cpp:276]
UE4Editor-Core.dll!FDebug::OptionallyLogFormattedEnsureMessageReturningFalse() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\private\misc\outputdevice.cpp:385]
UE4Editor-Engine.dll!TConsoleVariableData<int>::GetValueOnGameThread() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\public\hal\iconsolemanager.h:761]
UE4Editor-Engine.dll!UWorld::MarkActorComponentForNeededEndOfFrameUpdate() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\leveltick.cpp:776]
UE4Editor-Engine.dll!USceneComponent::PropagateTransformUpdate() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\components\scenecomponent.cpp:424]
UE4Editor-Engine.dll!USceneComponent::UpdateComponentToWorldWithParent() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\components\scenecomponent.cpp:357]
UE4Editor-Engine.dll!USceneComponent::UpdateComponentToWorld() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\components\scenecomponent.cpp:400]
UE4Editor-Engine.dll!USceneComponent::SetRelativeScale3D() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\components\scenecomponent.cpp:846]
UE4Editor-Engine.dll!USceneComponent::execSetRelativeScale3D() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\classes\components\scenecomponent.h:91]
UE4Editor-CoreUObject.dll!UFunction::Invoke() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\class.cpp:4198]
UE4Editor-CoreUObject.dll!UObject::CallFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:484]
UE4Editor-CoreUObject.dll!UObject::execVirtualFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1842]
UE4Editor-CoreUObject.dll!UObject::ProcessContextOpcode() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1755]
UE4Editor-CoreUObject.dll!UObject::ProcessInternal() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:698]
UE4Editor-CoreUObject.dll!UObject::CallFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:608]
UE4Editor-CoreUObject.dll!UObject::execVirtualFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1842]
UE4Editor-CoreUObject.dll!UObject::ProcessInternal() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:698]
UE4Editor-CoreUObject.dll!UObject::CallFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:608]
UE4Editor-CoreUObject.dll!UObject::execVirtualFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1842]
UE4Editor-CoreUObject.dll!UObject::ProcessInternal() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:698]
UE4Editor-CoreUObject.dll!UObject::CallFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:608]
UE4Editor-CoreUObject.dll!UObject::execVirtualFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1842]
UE4Editor-CoreUObject.dll!UObject::ProcessInternal() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:698]
UE4Editor-CoreUObject.dll!UObject::CallFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:608]
UE4Editor-CoreUObject.dll!UObject::execVirtualFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1842]
UE4Editor-CoreUObject.dll!UObject::ProcessInternal() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:698]
UE4Editor-CoreUObject.dll!UObject::CallFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:608]
UE4Editor-CoreUObject.dll!UObject::execVirtualFunction() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1842]
UE4Editor-CoreUObject.dll!UObject::ProcessInternal() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:698]
UE4Editor-CoreUObject.dll!UFunction::Invoke() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\class.cpp:4198]
UE4Editor-CoreUObject.dll!UObject::ProcessEvent() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\scriptcore.cpp:1053]
UE4Editor-Engine.dll!AActor::ProcessEvent() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\actor.cpp:580]
UE4Editor-Engine.dll!TMulticastScriptDelegate<FWeakObjectPtr>::ProcessMulticastDelegate<UObject>() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\public\uobject\scriptdelegates.h:459]
UE4Editor-Engine.dll!FConstraintInstance::OnConstraintBroken() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\physicsengine\constraintinstance.cpp:1219]
UE4Editor-Engine.dll!FPhysXSimEventCallback::onConstraintBreak() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\physicsengine\physxsupport.cpp:505]
APEXFrameworkPROFILE_x64.dll
PhysX3PROFILE_x64.dll
PhysX3PROFILE_x64.dll
PhysX3PROFILE_x64.dll
PhysX3PROFILE_x64.dll
APEXFrameworkPROFILE_x64.dll
UE4Editor-Engine.dll!FPhysSubstepTask::SubstepSimulationEnd() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\physicsengine\physsubsteptasks.cpp:418]
UE4Editor-Engine.dll!TBaseRawMethodDelegateInstance<0,FPhysSubstepTask,void __cdecl(enum ENamedThreads::Type,TRefCountPtr<FGraphEvent> const & __ptr64)>::ExecuteIfSafe() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:582]
UE4Editor-Engine.dll!TGraphTask<FDelegateGraphTask>::ExecuteTask() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\public\async\taskgraphinterfaces.h:797]
UE4Editor-Core.dll!FTaskThread::ProcessTasks() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\private\async\taskgraph.cpp:539]
UE4Editor-Core.dll!FTaskThread::ProcessTasksUntilQuit() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\private\async\taskgraph.cpp:340]
UE4Editor-Core.dll!FTaskThread::Run() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\private\async\taskgraph.cpp:690]
UE4Editor-Core.dll!FRunnableThreadWin::Run() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\private\windows\windowsrunnablethread.cpp:74]
UE4Editor-Core.dll!FRunnableThreadWin::GuardedRun() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\private\windows\windowsrunnablethread.cpp:31]
kernel32.dll
ntdll.dll
ntdll.dll