I use 'synergy' to make different Linux and Windows PCs appear as one working space for my mouse, keyboard and clipboard.
The problem is that 'synergy' is not very stable, and apparently it sometimes makes Windows (XP, if that matters) "think" that Shift button is always pressed. The most annoying thing is 'Shift+ MouseClick' combination could be mapped to a special functionality by some applications (e.g. Firefox), and mouse becomes barely usable with them.
Rebooting the PC helps to unlock the Shift but obviously this is not the solution I'd like to use.