For people who play the video game Counter Strike online, it's hard enough watching your back at the best of times. In the fast-paced first-person shooter, there are always players with quicker reflexes or a sharper eye.

But at the height of its popularity a few years ago, people started to come up against other players with skills that were too good to be true. Games like Counter Strike and Half Life – another shooter that was very popular online – had a problem with players who used software cheats that steadied their aim or let them see through walls.

So in 2006, when the stakes were raised by an online competition with cash prizes, an unusual pair of referees were called in. David Excell and Bill Fitzgerald were mathematicians who had just spun out an artificial intelligence company called Featurespace from their lab at the University of Cambridge. Their software was very good at one thing: spotting weird behaviour.