It's happened to me three times.
1) Back when Dailymotion had video compilations of women flashing, I came across a video of two girls that I had known from high school. The video datestamp would have put it a few years into college. I knew it was them because they used their real first and last names in the video!! They were so snobby in high school that I never would have guessed they would flash for a camera, but there they were. The weird part was that I didn't care at al that I actually knew them, and both of them had boobs that were MUCH smaller than their bras let on in person.
I should bring it up at our next reunion...
2) I found pics on this site that I am 99% sure was a friend from my wife's grad school. I didn't say anything to my wife in case I was wrong. (But had I been 100% sure, then my wife would have love to have known about it.)
3) I found pics on this site of a college friend. The datestamp would have been correct, and I am pretty sure the pics were in her dorm room. 99% sure, but not totally sure.
In all three cases, I didn't get the rush one would expect from such a discovery, though.