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