Right clicking a horse with nothing in your hand will get you kicked from the server with an NPE internal server error and you are unable to join again without deleting your player file. No errors appear in the server console.
This is a log of logging in, right clicking the horse, and being kicked.
2013-04-18 20:57:27 [INFO] BranicYeti[<IP REMOVED>] logged in with entity id 23523 at (-205.87881043889624, 69.7000000178814, 147.77642677834177)
2013-04-18 20:57:27 [SEVERE] Reached end of stream
2013-04-18 20:57:27 [INFO] BranicYeti lost connection: disconnect.endOfStream
removing player mount