I have a problem using Gnome on Fedora 17 where, after a period of time, I cannot regain focus on any windows; that is, I can only see the desktop screen. If I Alt-Tab I can see the other windows, but if I try to switch to them it immediately goes back to desktop. My usual solution is to log out and log back in, which fixes the problem.
Edit: Ctrl-Alt-Left/Right does not do anything. If I do Crtl-Alt-Up/Down to move to another desktop it still does the same thing.
Edit 2: I can now replicate the problem, and it always occurs when Alt-Tabbing. I've disabled the "Coverflow Alt-Tab" extension; if I don't get it again, I guess there is a bug in this extension