Oh sooo many...
Years ago I was ASM for "Shirley Valentine" which was a one woman show. We had two sets, the first one was in a kitchen, and the second one was a beach. A beach with real sand.
People were curious about seeing if it was real sand, so they would come up to touch, grab, take. Me and my backstage crew had to stand there and say "It is real, and we ask that you do not touch the sand".
When I saw a woman reach out I said the above, and she said "Oh but I have to". And then proceeded to caress the sand. She then tried to have a discussion with me about between the similarites of this show and "The Heidi Chronicles". I said "Uh, I don't know". She was upset that we didn't have a full blown literary discussion while she was petting the sand that she walked away.
Another time I used to work summers out at Tanglewood, where they have concerts. People can purchase season lawn passes, which you can sit on the lawn for free for all the concerts.
One person took it too far and thought it was an actual backstage pass. After the concert the crew was busy getting large instruments offstage when he would come back and flash his season pass at us and walk right onstage with his friends.
When we tried to explain that it was not a backstage pass he would argue that he was allowed to do this because he had a lawn pass. The crew would then try to run him over with a piano.