Admit you were in the wrong and pay up.
HAHA! Good one.
I think they take photos these days so if you say it fell down, best to say it was down on the floor I reckon.
I got a fine after forgetting to put a ticket on (it was free for a few hours anyway and I was only there about 30 mins). I saw someone going back to their car so asked them if I could have their ticket and sent it off with a letter saying it had fallen down and they let me off.