Comment 217 for bug 124406

That's very difficult for me to guess, legolas... But the mailing list thread quoted on http://bugzilla.kernel.org/show_bug.cgi?id=9147 seems to indicate that modifier keys such as Ctrl and Alt were handled the same way in 2.4 as all keypresses are in 2.6; that is, they weren't released until a keyup event was registered. For me at least, it's primarily the Ctrl key that's creating a problem in the first place, although there are a few glitches involving the mouse or other keys from time to time.

As mentioned in the other thread, though, if the bug you're experiencing is bug #9147, you should be able to temporarily clear it up by hitting Ctrl-Alt-F1, which somehow resets the keyboard driver, perhaps by attempting to switch from X to the console. It's just a way of working around the bug, of course, but it's much, much quicker than exiting all one's applications and restarting X.

If, on the other hand, this trick doesn't work for you, there's a chance that we're dealing with more than one bug.