people on my server crashing

my friends are playing, and they dont really do much like commands,etc.. they're just walking around
in the console i get this message and they disconnect

Code:

2011-05-21 20:25:35 [SEVERE] java.net.SocketException: Software caused connection abort: socket write error
2011-05-21 20:25:35 [SEVERE]     at java.net.SocketOutputStream.socketWrite0(Native Method)
2011-05-21 20:25:35 [SEVERE]     at java.net.SocketOutputStream.socketWrite(Unknown Source)
2011-05-21 20:25:35 [SEVERE]     at java.net.SocketOutputStream.write(Unknown Source)
2011-05-21 20:25:35 [SEVERE]     at java.io.DataOutputStream.write(Unknown Source)
2011-05-21 20:25:35 [SEVERE]     at net.minecraft.server.Packet.a(Packet.java:120)
2011-05-21 20:25:35 [SEVERE]     at net.minecraft.server.NetworkManager.e(NetworkManager.java:108)
2011-05-21 20:25:35 [SEVERE]     at net.minecraft.server.NetworkManager.d(NetworkManager.java:231)
2011-05-21 20:25:35 [SEVERE]     at net.minecraft.server.NetworkWriterThread.run(SourceFile:90)