I was experimenting with horses in the newest snapshot when I rode one into water that was 3 blocks deep. Doing this automatically removed me from the horse, and I tried to remount said animal a couple of times underwater. Doing so left me unable to place blocks and it made the system animate my character's lower torso as if I was still riding a horse. Saving and quitting the world resolves the issue, but transports me back to the point where I first tried to mount the horse.
I made a video showing the phenomena here: http://www.youtube.com/watch?v=k9VCja2fGys
(Skip to 2:20 to see the bug demonstrated.)
Steps to Reproduce:
1. Ride a horse into water more than 2 blocks deep. (I was in creative mode, I am not sure if this makes a difference.)
2. Attempt to remount the horse several times while under water.