Slimes (and Magma Cubes) can damage the player during their death animation where other mobs like a Zombie can't. 

I was able to replicate this issue in my creative world too which is where I got the attached clips from.

To create the issue: 

1. Spawn in a medium or large Slime (I tested small Magma Cubes too)
2. Kill the mob
3. As it's dying, walk over it and you should take some damage

The proper fix for this bug should be to prevent dead slimes and magma cubes from damaging the player. (As a player, being damaged by something you defeated feels wrong.)