The common problem with basic Tripod plates (quick release plates) is that you only have one main screw that attaches to the camera. So obviously with a little force, the camera seems to twist around, and sometimes come loose from the QR plate. This is especially more annoying when using a follow focus system and the lens moves away from the gear. If you look carefully under your camera, you'll find a tiny 'alignment' hole that most QR plates don't utilize.
To solve this problem, you would use an Anti Twist plate. Below are a few images of a Canon 5D Mark II Anti twist plate (only available for 5DM2 currently).
3 screws to fasten the plate and 2 thread to fasten a quick release plate
The Canon 5D Mark II Anti Twist Plate takes advantage of the alignment hole on the camera to make sure it doesn't shift, and then there are two threaded holes that you would use to mount your QR plate. Yes, you would use both threads in your slotted plate which will prevent the plate from shifting also. If you have the need for a firm stance, check out these anti-twist plates via eBay (click here).
Canon 5D Mark II Anti-Twist Plate