I remember in UT99 for years always running into situations where my aim was slightly off in situations where I was dead sure it should have hit. Turns out it just used the mouse acceleration feature in windows: the speed at which you move the mouse influences how far the crosshair (or cursor) moves. Once I disabled that I became about 5x better. The next big jumps were turning off vsync (and making sure it doesn't turn itself back on) and going back to CRT from LCD.