On a recent non-commute ride, I was climbing up a hill against a strong headwind and a rider snuck up behind me unannounced and drafted me up the hill. Near the top, he took me by surprise, overtook me, and said "thanks for breaking through the wind for me", and left me in his dust. I was too gassed to say anything, I just laughed it off. I thought that was kind of rude, but nothing I could do about it. I had no idea he was behind me because I was too concentrated on fighting the wind and the climb.