It happens, but in my experience the best thing you can do is - Smile Big, Wave Big (all 5 fingers), and if your feeling boisterous belt out a big Howdy, Hi, Hello, or Thanks!!!

Why... b/c

1. You don't give them the satisfaction of being mad
2. Maybe you get under their skin for not being upset and they are thinking "no you idiot I wanted to scare you"
3. You tend to end up laughing at yourself for being a happy ass and truly laugh it off
4. You spread the cycling love or whatever you call it