After compiling my project today, it successfully compiles but then triggers a breakpoint with this error message: UE4Editor.exe has triggered a breakpoint. [2014.06.27-19.38.44:311][ 0]LogWindows:Error: appError called: Cast of FPSCharacter /Script/FPSMMO.Default__FPSCharacter to Level failed […

After extensive experimentation, I found that the error came from trying to make a sub-object from a subclass of actor.