To avoid knocking down your friends instead of your opponents
Try right clicking your intended target. Once you have the correct target (their portrait will show up) select knockdown from the drop down radial bar. It's an extra step but once you get used to it its not bad. This could probably work with other advanced attacks which require a mouse attack.
Automatic Attack of Opportunity combat effect dispelling your invisibility/greater sanctuary
Tired of accidentally hitting a badguy with an attack of opportunity while you're trying to run around invisible/g-sanced? First, use WASD on the keyboard to move around instead of a mouse click, and second, pull out a ranged weapon. You can't make attacks of opportunity with a ranged weapon, so as long as you don't actually mouse click a hostile, you'll never accidentally un-invis/un-g-sanc yourself.