Since the 1.9.1 update my character has been intermittently hitting for negative 1 billion.  When these hits occur, the mob is immediately defeated and flies off really quickly. I have attached a screenshot of the damage, and below is a link to the corresponding capture (since it is 50+mb and above the limit):

https://streamable.com/xtyjt0

The capture shows two instances of this happening. 

There doesn't appear to be any particular actions that effectively reproduce this issue, but it does happen on every level, with any melee weapon.

The stats at the end of each level are also then scuffed, showing the same negative 1 billion+ damage, but this time as my damage dealt.