You're Not The Only One In The House
When I shot the Wilton/Fairfield Warde game, there was another photographer there also snapping pictures. It was the photographer who was taking pictures of the Seniors with their parents, so I'm guessing she was a hired gun. No problem, you stay out of my way, I'll stay out of yours.
But when the action started, something was very wrong. She was popping an off-camera flash right into the player's eyes! OK, I get it, you want to get good photos, but come on!
Besides the point, your flash ruined one of my opportunities because I was shooting in Natural Light! I brought the exposure down two stops in Lightroom and realized it's not a great picture anyway, so I didn't worry about it.