Confirmed, by spawning a horse by egg, and then right clicking it.
You only need to delete your Player.dat in the Players folder of the World, no need to delete the entire world.
Whenever I try to login to the server, I get the following on the server console:
2013-04-18 21:29:36 [INFO] MichaelGrue/xx.xx.xx.xx:50380 logged in with entity id 12840 at (287.4667900592452, 90.7000000178814, -185.42269998861883)
2013-04-18 21:29:37 [SEVERE] Reached end of stream
2013-04-18 21:29:37 [INFO] MichaelGrue lost connection: disconnect.endOfStream
removing player mount
Client gives:
Internal exception: java.lang.NullPointerException
Confirmed, and can also be duplicated by the following steps:
On Windows 7 (and probably 8) you can move your windows by holding the Windows key (also known as the Start key) and using the arrows to move windows. If you move the window to the far left or far right. And then Alt-Tab away from the launcher, and then Alt-Tab to get back - Window location and size is reset.
This does not happen when maximizing it as stated in Item 2 above.