When I spawned a horse (through the creative inventory), I attempted to ride the horse without doing anything else (applying a saddle, etc.)
The client only gives the following:
Client> java.lang.NullPointerException
Client> at ek.n(SourceFile:174)
Client> at ck.a(SourceFile:237)
Client> at ck.a(SourceFile:225)
Client> at ck.h(SourceFile:175)
Client> at ck.d(SourceFile:16)
Client> at cm.run(SourceFile:115)
Server gives this:
2013-04-18 14:11:28 [SEVERE] Reached end of stream
2013-04-18 14:11:28 [INFO] GrandTheftGamer lost connection: disconnect.endOfStream
Related issues
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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

Duplicate of MC-13628, please use the search function to see if your bug has already been submitted. Currently over 54% of tickets are being closed as duplicate.
This also affects pigs (both wild and spawned with spawn egg) when trying to ride them with after a saddle (spawned) has been placed on them.
Edit: I've got this issue on 2 accounts on a server that had gamemode 1 set. The horses are ridable in SSP creative.
It should be noted that I had to create a new world in order to play again.