Me too. Random, but often crashes. All worked smoothly before update to 4.12. I have ticks enabled (and don’t even use it manually) only in AI Controllers.
[2016.06.14-14.01.47:133][795]LogWindows:Error: === Critical error: ===
Assertion failed: AllEnabledTickFunctions.Remove(TickFunction) == 1 [File:D:\Build\++UE4+Release-4.12+Compile\Sync\Engine\Source\Runtime\Engine\Private\TickTaskManager.cpp] [Line: 872]