Never had an issue as long as the flash does not go off or you have a snapshot sound on your camera(turn that off too). I doubt they will buy the "I did not know the flash was on." or "I forgot to turn the flash off."
If your flash goes off, you pinpoint yourself for a tap on the shoulder. Consider courtesy for those behind you. Take your shot and then give it a rest for a little while. Try not to block others view.
I have used those simple rules and like I said, never had any issues.
Good luck with indoor, no flash photography. It is a challenge