Eating on com is one of my biggest pet peeves. Unlike Dam, though, it was my ME during a show. He had no reason to have his mic keyed, but every night after repeated requests for him to stop it continued until one night I went into his booth and took away his chips.
At our school, it's really important for clear com to be very formal. For some reason, they designed our theatres with clear com panels in the dressing room and the green room. There is no built in mic in these panels, but anyone can turn on the panel and listen in to the conversation. This came up when one of our light designers was using com to communicate with his ME, and started complaining about sex with his girlfriend. That was mighty interesting.
Even with the importance of formality, its still hard for many of the crew members. During one of our musicals, the stage manager forced a quiet com during her favorite song, and she would sing along and cry with it. (She also seemed to have a lot in common with Max's stage manager -- except she liked to talk about her "female problems") During our production of The Marriage of Bette and Boo, the stage manager would remove her com when there were no cues due to crew members refusing to stop telling dead baby jokes.
Like Cat's outdoor, I work one where we have walkies, and unfortunately the company is too cheap to buy headsets for all of them. During our tech last summer, we ran into the similar problem of the SM bad mouthing certain people, but she continued even after we informed her about the open mic problem.
One problem I had on the other end of the spectrum was an ASM who never was on com when I needed her. Through out the run I asked her to be on com at certain points, and I always seemed to have to send a crew member to run off and find her. She was a wonderful ASM in all other aspects, but I had no idea how to get her on com when I needed her. And then, when she did finally come on com, she never announced it. So frustrating!