I've only experienced one. We were landing at ORD 25 years ago, but I remember it like yesterday. We were maybe a hundred feet over the runway, when the pilot pulled up and came around again. He didn't tell us what was up until he was ready to try again. Another a/c had crossed the runway we were landing on instead of holding up. I would have loved to have had access to channel 9 to hear our pilot cuss out the offending pilot.