I had an interesting experience departing FRA last year. At departure immigration, they scan my passport and the computer says something to the effect of "no record on file." The officer gets a furrowed look on her face, and then starts flipping through the pages looking for my entry stamp (I assume that what normally happens is the computer has a record of your entry into Schengen regardless of which country you entered through, and assuming you haven't overstayed, they clear you for departure without necessarily finding your entry stamp).

Well, I had no entry stamp. Something odd had happened upon my arrival at CDG. I was whisked past the immigration line by an airline employee, and she showed the immigration officer my passport cover (US passport) and said something in French to him, and he just waved me through. So, no stamp in my passport and no entry in the computer that I had been lawfully admitted. I didn't think much of it, as the whole int'l arrival/departure process in Europe has seemed sloppy to me as compared to other countries.

Instead of just playing dumb, I spoke up and told her that she would not find an entry stamp even before she finished paging through my passport, and explained that there was some mix-up at CDG and they didn't stamp it. She paused for a moment, looked me in the face, and I could see on her face that she was thinking "Oh, the (gosh darned) French!" for a moment, and then she sighed and waved me on. Rather than fix the problem, I think it was easier for her to just ignore it and let me pass, so there's probably no record of either my arrival or departure in the Schengen computers or my passport.