I was having some problem with my mouse, when I tried to move it in a horizontal line on the screen it kept going at the wrong angle, like it would go down or up too much (not by a huge amount but enough to notice, seemed a bit random though).

I thought it was my new mouse and its sensor position / firmware calibration or some shit, and thought I'd just get used to it. But no, I didn't get used to it and it was starting to piss me off. So anyway I looked this kind of thing up on the net and found... stuff... anyway from that stuff I found out the windows 7 / vista aero pointers can mess with your mouse in strange ways. So, I went into the windows mouse settings and set the pointer scheme to "none" and it fixed it...!

I don't know if its specific to my mouse (roccat kova+) or anything, I heard something about graphics card drivers.. but whatever. I don't trust the aero pointer scheme anymore.

This even affected ql btw, even using in_mouse 2.

So if you have this problem then try that, it could be your solution :)