6
The scratch cards that could be read from outside
A statistician worked out which unscratched cards were winners, on the printed numbers alone.
In 2003 Mohan Srivastava, a geological statistician in Toronto, was idly looking at a tic-tac-toe scratch card and noticed that the visible numbers around the latex were not random, they encoded whether the card was a winner.
He tested it by sorting a batch of unscratched cards and predicted winners with about 90% accuracy. He then worked out he could make roughly $600 a day doing it, decided that was less than his consulting rate, and phoned the lottery instead.
It took him several attempts to make them take it seriously. The game was pulled. The general flaw, visible data correlating with hidden data, has been found in scratch games in more than one country since.