For those lucky ones who haven't encountered this bug yet, here's how it works: when clicking any mouse button in WoW, the mouse cursor often jumps to the center of your screen. If this happens while moving / camera panning, it will cause you to jerk around and change direction randomly (i.e. instant involuntary 180° turns etc.), which is of course a huge disadvantage as it can make you run off cliffs and hinder you when trying to select a target.
This bug has been around for as long as I can remember. However, the frequency is what changed. It never happened often enough to really be more than a slight annoyance, but recently it's been happening about once every 10 clicks, which makes it almost impossible to play properly (especially PvP).
At first I thought it was caused by Win 8.1, which was partly true. Win 8.1 seems to have a problem that causes this bug in a lot of games, and it happened with almost every click in WoW. I've installed the fix provided by Microsoft, which brought it back down to about 10% of clicks. However, a friend of mine using Windows 7 has the same problem.
It also happens with no addons on a fresh installation, as well as on a completely different machine. I've also tried various other supposed bugfixes I've found on the official forums (disabling TabletInputService, updating drivers, using different cursor settings (hardware cursor, input lag, ...) and what have you) but so far nothing has helped.
And of course this happens exclusively in WoW.