Either for not wanting to pay, being too young, etc.
I snuck into South Park's film back when it came out in '99. I was 12 at the time and though I made it through the whole film without any staff spotting me, I had butterflies in my stomach the whole time. If I recall right there were a few other kids around my age too.
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Moviehopped all the time. I always snuck in McDonald's too.