I was just trying the latest version of Minecraft (snapshot w18a50). Well I was fighting a skeleton out in the rain, and it shot and killed me. When I respawned, I could tell something was wrong. First, I noticed that my sword was not laying on the ground where I'd died. It was gone, which shouldn't happen because I'd just dropped it a few seconds before (not long enough for the game to decide that it should despawn in). Then I noticed I couldn't see the skeleton, but it could sure see me. And it shot me again. The arrow it shot looked like it had spawned in the middle of the air and went flying at me, because the skeleton model itself was invisible. Now I am not sure if the unique set of circumstances to trigger this bug require rain or not, but it was raining when this happened, so I thought I should mention it.