Mouse Acceleration is a setting that is enabled by default in most operating systems. Essentially, what it does is register your mouse as moving further whenever you move it quickly.

This can throw off your aim in tense situations where you jerk your mouse around a lot, since it makes you aim more than you need to. Some people have reported luck using custom mouse acceleration programs, but I personally recommend completely disabling it on any computer that you intend to play Team Fortress 2 on.