NPE when trying to ride horse

NPE/Crash when trying to ride a horse. Java traceback as follows:

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)
Client> 2013-04-19 23:40:31 [CLIENT] [INFO] Connecting to 192.168.100.97, 51934
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)
Client> 2013-04-19 23:40:57 [CLIENT] [INFO] Stopping!
Client>
Client> SoundSystem shutting down...
Client> Author: Paul Lamb, www.paulscode.com
Client>