Using NetBeans 6.7 RC1:
When using the Sybase JDBC driver (jconn3.jar) to connect to a Sybase server, and leaving the connection unused for a
while (varies depending on the server configuration I guess), the server will automatically close the connection and the
next time a JDBC command (such as SQL) is issued, it replies with a "Connection already closed"-exception.

I had an open connection in Netbeans (on Service tab, under the Databases node) unused for about 5 minutes. Then I
Right-clicked and chose "Disconnect". The server had already closed my connection and NetBeans informed me of the
Exception via a red icon in the bottom right corner. However, the tree under the server node  was still fully populated
and the icon was still that of a connected connection. The right click menu was changed to enable Connect and disable
Disconnect though. I guess somewhere in the code, maybe the Connection.close() isn't guarded for a SQLException and
therefor never clears the tree. 

Or maybe it's supposed to work like this.