In the 13w06a snapshot, the death messages are wrong. For an example, if you die from lava it just says "tried to swim in lava", without any name (Picture 1). And if you get killed by a mob, the same thing happens except the name of the mob is replaced by your name (Picture 2 and 3).
This doesn't seem to affect all death messages though, since dying from fall damage creates the normal death message. There might be other ones working fine as well, but I haven't tried everything.
How to recreate:
Just go into survival mode and let a mob kill you.
Duplicate of MC-9115, please use the search function to see if your bug has already been submitted. Currently over 51% of tickets are being closed as duplicate.