I've heard people argue that either they were never even on the road they supposedly sped on and that perhaps the camera got the license plate wrong or that a car ahead/behind them was speeding and the camera caught the wrong person. Whether there's ever any truth in any of that though I have no idea.
I always fail to see how this can be argued. A machine that fines you for enforcing the law. If you stay within the law, you don't get fined.
rants about how wrong speed cameras are