First time I logged into DayZ stable branch since .51 rolled out, and my character immediately starts punching and does not stop. Disconnecting and reconnecting to the same server did _not_ stop the animation, nor did closing and reopening DayZ or fully rebooting my PC.
It is not just the animation, but also the sound, and the effect of damage seems to be applied (I stood in front of a zombie, while attacking animation played, and it was staggered and appeared to bleed, as when melee attacks land usually... I did not kill it so I don't know if damage is actually applied).
I came across another player who confirmed I was endlessly punching, so the animation bug is also serverside.