When you try to set the Play Rate Track to less than 0 the editor will crash with this.
[2016.05.28-10.54.55:463][331]LogWindows:Error:
=== Critical error: === Assertion failed: GameDeltaSeconds >= 0.0f
[File:D:\Build++UE4+Release-4.12+Compile\Sync\Engine\Source\Runtime\Engine\Private\LevelTick.cpp]
[Line: 1130]
All you need to do to reproduce it is create a Level Sequence asset, add a Play Rate Track and try to set it below 0. This was done in a clean project.