Yep, I've had that happen too. During the time I raced bicycles, there was a roller blader team training for the PanAm games in Winnipeg. One of the members of the team also raced bicycles - she and I knew each other. I'd go to a particular park with a 3 km loop and would be riding round and round doing intervals or whatever, when all of a sudden I'd have a line of roller bladers drafting me, with my acquaintance right at the front.