I searched but found nothing, but if someone has posted about this, please ignore it. Anyway. I launched a new server on the new snapshot. Me and my friend got on and wanted to try out the new horses. When we "ride" the horses, the player who wanted to ride it, got kicked. The kick message is: "Internal exception: java.lang.NullPointerException". In the launcher, it says

Client> 2013-04-20 02:06:22 [CLIENT] [INFO] Connecting to 184.153.70.96, 25565
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)".

Now, when I try to join back into the server, it instantly kicks me again with the same error. In the server console, its marking it as a End of Stream.. as shown below.

[INFO] Zelakto[/184.153.70.96:57230] logged in with entity id 13716 at (-134.5, 3.699999988079071, -655.5)
[SEVERE] Reached end of stream
[INFO] Zelakto lost connection: disconnect.endOfStream

Thanks for your support!