1. A few years ago, some friends and I went to a Ben Folds concert. We got there early so we were right up by the stage, but there were hours of wait and cover bands. They ended up packing us tight as sardines in there. So the girl next to me started to chat to me in the hours while we waited. At first she seemed nice, friendly, and normal. Then she started saying things that let me know she was obsessed with Ben, that she spent all her money following him around on tours and being a fan.
Then, it gets weirder. She told me that she had taken a page of the music to her favorite song of his, had it laminated, and sewn it into a wallet, which she showed me. She explained that her plan was to give this wallet to him and that she just knew he would cherish it, etc. Meanwhile I'm thinking, this girl has a few screws loose!
Ben got on stage. He plays piano and sings, so it's not a passive job. In the middle of one of his songs, nutty girl takes out the wallet, tries to climb up on stage, and FLINGS THE WALLET AT HIM while yelling, "I love you, Ben!" Yes, she threw it at him while he was in the middle of singing and playing piano! I was horrified. Ben was a pro -- he just literally ignored her, and ignored the wallet sitting on the floor of the stage for the entirety of his performance.
2. I'm a proud RENThead

I have only ever seen good fan behavior there. The setup outside the theater was perfect for this. There are doors at the back where the cast exit. Imagine a square -- 1 is the wall of the building including the door where they exit, 2 are ropes that keep the fans back from the sidewalk directly outside the exit door, and one is the car waiting to take the star away. So a star that wants can exit the dressing room, wave at the fans, walk to the car, and drive off. A fan who wants to can stay and sign autographs or take photos. Fans have to remain behind the ropes. So I did go a few times and wait for hours behind the ropes for an autograph, but I didn't think I or anyone there did anything rude. We took what attention we were given, didn'ty harrass the actors, and din't throw a fit for the stars who didn't stay. And it was really fun camping out with fellow fans. No professional autograph hunters in sight!
BUT
Once when I was seeing RENT - for the last time on Broadway, actually

- I ended up seated next to a strange girl on my left. She told me (what am I, a magnet for random strangers to talk to?) that her favorite show was actually Spring Awakenings and that she camped outside their entrance EVERY DAY. And every day she brought gifts for the entire cast, for example today, she gave them all coloring books and crayons, isn't that awesome? Um. It took all I could muster to prevent myself from saying "Heck no! Not awesome at all!"
She also insisted on commenting to me during the entire plot of the show, as did the lady to my other side. All 3 of us had seen it multiple times before so it wasn't like gasping "Oh!" when a surprising thing happens, which would be fully understandable. It was more like they forgot they were at a live performance and wanted to gossip to me about the actors and the songs and the plot, in real time! aaack.