What Billy Ripken has become most famous for his is 1989 Fleer card #616. It's not a rookie card or anything. 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.