Serge Humpich
French engineer who showed in 1998 that the signing key on French bank cards could be factored on a home computer, and got a suspended sentence for his trouble.
French chip cards carried an RSA signature meant to prove the card was genuine. Humpich found that the key was only 320 bits long, factored it, and could then produce cards that terminals happily accepted. To make the point he bought Paris metro tickets with a forged card, then approached the card consortium GIE Cartes Bancaires to negotiate over what he had found.
The consortium responded by calling the police. In 2000 he received a ten-month suspended prison sentence. The publicity was followed by forged "yescards" that answered yes to any PIN, and the banks moved to longer keys. The security had effectively rested on nobody bothering to do the arithmetic.