During the current snapshot I had gotten killed by a Skeleton, and when the day rolled around and everything burned and died (or went for cover) I found I was getting shot at by something. I looked around but saw nothing, other than some arrows coming from the water. I couldn't investigate since I was in no condition to, so avoided it.

Later I went looking, but couldn't find the skeleton. So I checked in Spectator Mode so I could look safely, and found nothing.

Much later I had gotten killed by mobs while hunting, and when I came back for my stuff they weren't there.... Or so I thought. They were still there but invisible. When I saved and closed the game and relaunched it, they were all visible again. As were the sheep I was looking for prior to that.

Curious, I started a new Superflat game to test something.  I spawned a lot of zombies and let them kill me. Upon respawning, they were all invisible (Attached image). They could see me just fine though.