I love you number guys. But it's always a math professor.
I'd understand if it was a test day, and some students were squeezing out those last couple minutes to check their work. I'll cool my heels for that, because I get it.
But it's usually not. It's usually "Just one more problem." Or "Just one more question." Or "Let's sit here and do problems so we're the only two people in the room."
Meanwhile, my students and I are lined up in the hallway, waiting with increasing impatience. And when they finally leave, I have to smile and say "Oh, no problem," while I try to log onto the classroom management software (which takes a good four minutes to boot up, four minutes of class time now because you didn't let me in the room), get my preliminary boardwork done, and arrange handouts.
It is not okay. I am being polite to model that respectful behavior for my students, but it's not okay. You've taken my time. You've taken their time. Class started, officially, two minutes ago, but your math problems were more important than my skills and content area, apparently.
And don't give me "Oh, time got away with me." The big hand counts off minutes. The little hand, hours. My class? It starts at 9:00 AM. Yours ends at 8:50. This is easy. Big hand two big marks from straight up, you say "we'll continue next time" and pack up your markers and feelies and whatever else you use for math these days.
And it's always, for some reason, a math professor. And moreover, it's always -- every single time -- an adjunct. I used to be an adjunct. If you're giving extra free labor to the university, you're a chump. We don't pay you enough for that. Pack up, go home, and let me get on with my class.