In 2009, we were on a flight LHR-BOS, with our twins, who were then a month old. We had a nice conversation with the couple behind us, one of whom turned out to be a local pediatrician. Two years later, the same couple was again behind us. They didn't recognize us but had actually recognized the kids, even though the kids were infants when the couple first saw them.
Shortly after my wife and I got engaged in 2001, her former boss invited us out to brunch (He had introduced us.). We were living in NYC at the time, but my parents came in from Boston. Her boss brought a date. The date and my mother started chatting, noticed an unusual number of coincidences regarding their backgrounds and, after about 15 minutes, realized that they were third cousins who hadn't seen each other since they were kids at camp.
Mike