http://www.youtube.com/watch?v=reNPK8XD-k4

This video fully explains how players on wifi or players with bad internet connection ruin the game for everyone
In the picture...you can see on the left which is what you see but on the right is what is happening with a laggy player and lag compensation. On their screen you didn't even start shooting yet because you run 20-50 milliseconds behind what they see