Only once have I had an issue with someone at an event - and that person was the referee at an U11's rugby game my daughter was playing at.
I had the E-3 on a Monopod, with spare E-510 around my neck and had (as I always do) informed the opposition manager and parents that I was asked to take shots and was the official tog of my daughters team, and that they were more than willing to peruse all shots on my www site and all proceeds go towards to the club. Never ever had any objections, only requests for images.
Anyway, game started and 5 mins in, the ref spotted me, blew up, stopped the game and came within 15 yards shouting over... "Excuse me sir do you have permission to take these shots and have you spoken to Mr 'coaches' and parents", I replied that yes I had, but nobody had asked me if I mind video of my daughter being taken, or if I had objected to other parents with P&S cams taking shots and was he going to ask the same questions of those parents videoing and taking shots. I told him to just get on with his job. The crowd around me laughed under their breaths as he was firmly put in his place.
Idiot.