When trying to join a multiplayer server that is running the latest snapshot, I always get this error in the launcher:

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> 2013-04-18 17:21:17 [CLIENT] [INFO] Connecting to (server ip) , 25565
Client> 2013-04-18 17:21:25 [CLIENT] [SEVERE] Reached end of stream
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> 2013-04-18 17:21:30 [CLIENT] [INFO] Stopping!
Client>
Client> SoundSystem shutting down...
Client> Author: Paul Lamb, www.paulscode.com
Client>

(sorry if this is a bit spammy.)