What big mistakes did your teachers / professors make?
1. Nobody's Asking for a Show, But Keep Me Awake
A bigger point might be: "Participated in and supported the academic system that perpetuated the big-lecture weed-out system for certain majors".
Basically, droning on to a lecture hall of 200 students for hours on end is hardly a way to educate anyone.
2. Relied on Grad Student TAs to "Solidify" Content in Recitation
The fact that they had to have anything resembling a "redo" for a supposedly valuable educational procedure already smelled like a joke.
Most of the TAs were useless and uninterested in being there to help the undegrads. The few that were couldn't get enough time in with the truly engaged.
I had one guy write math problems on the board and he was so utterly moronic he erased half of them by leaning on the board with his shirt before we could finish copying them down.
He didn't go back and fix it or redo the problem. Just bowled on through the rest of the content.
3. Accused Me of Plagiarism
But, after I proved I hadn't plagiarised my paper, admitted it was the best writing she'd ever read by a student.
I still have the paper. It was pretty good, I suppose, for a 17 year old. A little cringey at this point.
4. Spent More Time Handling the Disruptive Kids Than Teaching the Willing
Not a fault of the teacher, but more of that of the system they are forced to operate under. This happened more in the lower grades (US-based) than high school.
5. Forced Useless Trend-Setting Technology On Students
There was a laptop pilot program in the early 2000s when I attended high school. I opted out because I thought that nobody at the school had a shred of evidence that forcefully introducing laptops into the curriculum would bear any kind of results for our education.
For my stance against stupidity, I was rewarded with being held out of certain honors classes due to "scheduling" because "those classes required participation in the laptop program" in order to part of my registration.
I won in the end. They disbanded the program 3 years after starting it.