I remeber seeing a show of Derren Browns where he went to a dog track and bet. If he had a losing ticket he went to the cashier and said it was a winner, the cashier paid out on a losing dog evein though all the tickets were full of barcodes!
How was this done, anyone know?
Not that I am going tothe track this week, it is one of those things that has bugged me for yars

Have a look at my reply to this thread:
http://community.channel4.com/eve/forums/a/tpc/f/5006034991/m/5670049844
for a vague idea as to how he did it.