Mobs in water will sometimes appear to swim down, and then suddenly teleport back up. The server and client seems to disagree on where the mobs really are, and the teleport is a syncronization event.
Demo video: http://youtu.be/LFDxpokPbEU
Only tested on multiplayer server.
Linked issues
Comments 3
Resolved as duplicate of MC-1241
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.