Fast cyclists who you don't hear coming up behind you who just pass you in zooming close proximity without saying anything or giving you warning that they are coming up behind you on the road. They give me a heart attack, they're lucky I don't swerve for some reason right as they are passing since I don't know they're there, and I just think it's pretty rude not to let people know you are coming up behind them and are about to pass you fast. Is it too much trouble to call out "Good morning!" as you are approaching behind someone?