When I click on a horse to try to ride it, it crashes with:
Internal exception: java.lang.NullointerException
and this on the log:
Client> 2013-04-19 21:21:57 [CLIENT] [INFO] Connecting to localhost, 15565
Client> 2013-04-19 21:22:06 [CLIENT] [INFO] [CHAT] Your game mode has been updated
Client> 2013-04-19 21:22:21 [CLIENT] [INFO] [CHAT] Your game mode has been updated
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)
Client> java.io.FileNotFoundException: http://assets.minecraft.net/1_6_has_been_released.flag
Client> at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
Client> at kq.a(SourceFile:237)
Client> at bhq.run(SourceFile:114)
Client> at java.lang.Thread.run(Unknown Source)

Note this is on a multiplayer server. Works fine on single player.
EDIT: On multiplayer it crashes when you try to ride any rideable entity.