I haven't bothered to look, but I bet this is documented somewhere in a 'worst practices' list...

I recently logged in to my PC (on a SBS domain) and found that I was not connected to the server (or anything else, for that matter).  I tried troubleshooting the PC's connections, rebooting, etc., but got nowhere. 

I ended up logging on to the server itself to see what was up, and found the old "Invalid System disk or disk error..." message on the screen.  It turns out that Windows (SBS 2003) had updated itself and forced a reboot, but couldn't finish the process because a floppy disk was in the server's drive.  Removing the disk was all that had to be done to fix the problem.