What Billy Ripken has become most famous for his is 1989 Fleer card #616. It's not a rookie card or anything. Yet, this Bill Ripken card is one of the most valuable 1989 Fleer baseball cards. Its popularity has to do with an unfortunate thing he wrote on the knob of his bat - F**k Face. Really, he only did it to make sure that he knew which batting practice bat was his. But a photo was taken with him holding that bat. Fleer didn't catch it before the set went to print and a PR disaster ensued.
CNBC has a full report about the obscenity card from 2008. There's also a website, billripken.com, dedicated to that one card, and it's still being updated! Several variations of this card exist, including a "clean" version and many others with various amounts of blackout over the obscenity on the bat handle.
Today, copies of this error card sell for $20 or more. It's perhaps the most famous "error" card in the history of baseball cards. If you're a Billy Ripken fan, and you happen to come across copies of his #616 1989 Fleer card, see if it's the error. If you don't want it, there are plenty of people that do.