Yesterday my game started crashing out of the blue. The output window had content that resembled this snippet from https://answers.unrealengine.com/questions/132817/attachment-replication-crashes-client-in-451.html :

[ 79]LogOutputDevice:Warning: Script Stack:Actor.OnRep_AttachmentReplicationAssertion failed: !bRegistered || AttachParent->AttachChildren.Contains(this) [File:D:\BuildFarm\buildmachine_++depot+UE4-Releases+4.5\Engine\Source\Runtime\Engine\Private\SceneComponent.cpp] [Line: 903] Attempt to detach SceneComponent 'Default Root' owned by 'BP_Bow_C_1' from AttachParent 'CharacterMesh0' while not attached. UE4Editor.exe has triggered a breakpoint.

In my case I had recently removed a component from my player pawn, so I think that was related. I double checked that things were running fine in the editor and that the pawn looked legit. I also made sure nothing more suspicious than usual appeared in the output log. Thinking the game and editor were somehow "out of sync" I started trying random things like rebuilding all from VS2013. It was the act of cooking the content in the editor, however, that made the problem go away.

So if you suddenly get random errors in DebugGame mode, try cooking the content from the editor to fix it.