Rude audiences, too many bad behaviors to name them all
Onstage:
Actors in the ensemble/background that start talking amongst themselves. I'm ADHD so stuff like that is ridiculously distracting and I find myself paying more attention to their antics than what the focus of the scene is supposed to be.
Actors that break character during really emotional scenes. If it's a comedic scene, I can deal with them laughing, but if it's at a freakin' funeral, it ruins it for the audience.
I also hate when actors phone in performances. If you're having such a bad day that you can't do your job properly, don't make the whole show suffer and just don't come in. If I wanted to see mediocrity, I'd just go to the local high school show, not pay $100 to see it on Broadway.
Like a firework unexploded
Wanting life but never
knowing how