Bug occurs in both game modes. Player spawned Horse using Spawn Egg in creative, then attempted to right-click to mount. Client disconnects with error Java.lang.NullPointerException; server reports:
2013-04-18 21:28:11 [INFO] 4am[/108.34.217.76:1024] logged in with entity id 1551 at (212.62373750401767, 66.7000000178814, 305.9407252902083)
2013-04-18 21:28:11 [SEVERE] Reached end of stream
2013-04-18 21:28:12 [INFO] 4am lost connection: disconnect.endOfStream
2013-04-18 21:29:24 [SEVERE] Reached end of stream

no stacktraces or errors are generated, it seems like bad entity info is written to the map, because players cannot log back in once this happens.

Attempted in both Creative and Survival.
Process:
(fresh map, vanilla 13w16a server, all brand new and clean)
1. Creative player named a Horse spawn egg using an anvil, then attempted to mount. Client kicked with NPE, did not crash. Player could not log back in.
2. 2nd player (logged in this whole time) spawned horse in creative mode (did NOT name using anvil), changed /gamemode 0, then attempted to mount. Same result.

Neither player experinced crash in SSP.