At our concert series we had a person who showed up a few times claiming that he had a "contract" with the performers to take photographs of them. Turned out he had emailed them in advance to ask if he could take some photos at their concert and they said "sure", but they had no idea who he was, just assumed he was a fan that wanted to snap a few shots. He was extremely intrusive, walking around with his camera during the concert, using the flash, snapping multiple photos, etc. We had a talk with him and he stopped coming around.

Unobtrusively taking a few photos is fine, but some people just don't seem to have a clue about how distracting it can be. I forgot the lyrics once in the middle of a song because someone kept snapping flash pictures from a close angle. It's not polite to the audience or the performer.