What error is this???

What does this mean???

Could it be to a source of my weird lag?
2011-06-25 23:22:51 [SEVERE] java.net.SocketException: Socket closed
2011-06-25 23:22:51 [SEVERE] at java.net.SocketOutputStream.socketWrite(Unknown Source)
2011-06-25 23:22:51 [SEVERE] at java.net.SocketOutputStream.write(Unknown Source)
2011-06-25 23:22:51 [SEVERE] at java.io.BufferedOutputStream.flushBuffer(Unknown Source)
2011-06-25 23:22:51 [SEVERE] at java.io.BufferedOutputStream.flush(Unknown Source)
2011-06-25 23:22:51 [SEVERE] at java.io.DataOutputStream.flush(Unknown Source)
2011-06-25 23:22:51 [SEVERE] at net.minecraft.server.NetworkWriterThread.run(SourceFile:116)

Its weird. Please help!

EDIT by Moderator: merged posts, please use the edit button instead of double posting.

 

You can get that from running some sort of server mod and those that connect don't have the proper mods to connect to your server. That's at the very least one cause, no clue about others.
But that should be evident when they join and are immediately kicked.

There is also a thread at the top of this section.
http://forums.bukkit.org/threads/socket-write-error-join-the-discussion.15644/