Didn't something similar happen at that Jump Fiesta you usually go to each year. I think Oda was drawing a picture and it turned out to be a rough sketch of Jinbei before he appeared in the comic as even a silhouette.
I always thought that was neat since Jinbei's pretty much been alluded to for about a decade I think and having what he'd look like be revealed in such a nonchalant manner.
I'm sure Greg will remember that, seeing as he actually presented a sketch done by himself (based on the one by Oda) here on the forums to us, and it was pretty close to the eventual design of Jinbei as well. I actually remember him getting "mocked" by some about how bad the sketch was then, but he had the last laugh.