In Boomsday Project Dr Boom Mirror 1/4 in The Secret Lab, there is a Worgen Greaser that needs to die to two abomination deathrattles in order for the puzzle to be solvable. However, the greaser has 5 health and consequently cannot be killed this way, rendering the puzzle impossible.
Looking at guides online, this apparently used to use the original 6/3 Worgen Greaser (and later 6/4) from Mean Streets of Gadgetzan, but as of yesterday’s patch appears to now use the Caverns of Time 6/5 Worgen Greaser.