Slime blocks make transparent entities in front of them invisible/look like they're behind them.

Just look at the pictures, because whenever I try to describe this glitch in detail I discover some other nuance to it. As far as I can tell, it only works for stained glass (not regular glass for some reason) on something's head, and mobs/entities which are supposed to be invisible but you're on the same teamor you're in spectator mode, so they appear transparent.