Here is what happened. Sorry for his wording. The GUID had been working perfectly until he changed his resolution (to 1280 by 1024, when his computer can only go up to 1280 by 800). This is from another site (knightsofreason.org)
"Recently, my GUID has "broken" and I have been trying to install a new one. The cause was when I accidentally set the resolution of the game to an output beyond which my computer can do. I have re-installed TJW's backport over three times, all of the backports downloaded independantly over three different times. I have even tried the svn backport... So, I re-installed trem. Without the backport, tremulous still works, but as soon as I use the backport, it won't function and I recieve an OpenGL error. So, I won't be on Tremulous with a GUID for a while....Hopefully by the end of the week I can have this fixed."

NOTE: This only appears when I use the backport.