Ticket refund and free pass scams (there was a hair/screw/you name it in my popcorn, I broke my tooth on a popcorn kernel, oh this stack of 9 tickets for different showtimes are totally all mine, and the rest of my party had to leave ) organized by small time crime families are far more common than anything else at busier locations, though you sometimes get groups by 4-8 teenagers breaking in through exit doors when someone leaves at large multiplexes.
Several theatres for example don't list film showtimes and film titles above auditorium doors anymore so it would be luck of the draw if you could figure out an auditorium with a Batman. Most theatres are so bareboned staffed that there isn't really a lot of staff able to watch for people sneaking around, though most theatres now do reserved seating so it gets pretty obvious if someone takes a seat that's not sold.