Encountering a "stuck key" condition - a key that's permanently active - can be highly frustrating. A control key permanently depressed can mean that mouse events behave improperly, such as the scrollwheel controlling zoom instead of page scroll, or left mouse clicks triggering right-click menus.
I occasionally encounter this condition on an Ubuntu configuration, which I expect is an artefact of Synergy usage, but don't have the time to debug.
To resolve this frustrating issue when it occurs, I use the following alias:
alias fixkeydown='DISPLAY=:0; for i in `seq 1 256`; do xdotool keyup $i; done; echo Done'