I find it's hard to say that there is a hard and fast line, it kind of falls under the terms one judge used when asked to define pornography, "I can't define it, but I know it when I see it."
I feel lucky in that, having worked with some people who had a bit of a reputation for being difficult amongst their respective circles, that I have only had one instance where I felt that somebody crossed the line. This was during a production of The Will Rogers Follies and we had an in-one scene change US of a black scrim while Will was doing his final radio monologue DS. The crew, being the first time we were doing it in the dark during tech, were understandably a bit louder than would have been ideal. The director went off on them about how this was a pivotal moment in the show and it could not be interrupted by such a disrespect for the scene. I quickly called a hold and a break, as my crew in this situation consisted of high school kids and volunteers who did not need to be subjected to that. The director quickly vanished and managed to keep out of my sight during the break, and I searched for her as I wanted to talk to her about it. Luckily once I called everyone back she reappeared and before I could say anything called the crew onstage and apologized for her tirade and gave a very sincere thank you for the work they had been doing during our tech period, as it was not an easy show and they had indeed been doing great with some fairly complicated scene changes and transitions.
In hindsight I'm very glad it happened the way it did, as I think the crew, while slightly hurt at first, saw that they were indeed valued and appreciated and seemed to throw themselves into teching the end of the show with even more gusto than before.