The prank goes something like this:

1. Identify someone you don't like
2. Find out that person's license plate number
3. Using glossy, photo paper - print out a duplicate license with that person's info on it
4. Tape it over your own license plates
5. Speed through stop light cameras field of view
6. Watch them get lots of fines that they don't deserve

BONUS - Impugn the credibility of stupid speed/stoplight cameras.