You walk around a park, wait for the sun to be reflecting just right off the water ... and ... someone parks right in your shot and starts a picnic.
You're shooting a running or cycling race with your 300mm f2.8 pointing right at the course and someone will walk right up and stand in front of you. ARE YOU BLIND? If this was a bazooka, would you stand in front of it?
You're standing on a small wall across the street from a stage during a major event. People are everywhere - watching, what else, the stage. So a family with three kids, all with balloons come along and park right in front of you! "Excuse me, can you hold down your balloons???" "Oh, are they in your way?"
Grrrrrrr ..... 


one old lady saw us from the bottom of the stairs and quickly took another direction 
