When trying to ride a horse on a 13w16a server, you get disconnected for Internal exception: java.lang.NullPointerException and it also leaves a error on the launcher. Also if you try to log back onto the server it will continue to do this.

Error:
Client> 2013-04-20 13:54:52 [CLIENT] [INFO] Connecting to 192.168.1.8, 25565
Client> java.lang.NullPointerException
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)