Here are some really dumb things that some teachers do.
Ignore Your Students
Abuse Your Students
Homework Returned Late (or never)
Don't Give Feedback
Sink Or Swim
One Way To Learn (Assume Your Learning Style is Their Learning Style)
Teacher Is Boss
Student Is Boss
Don't Prepare
Never Be Spontaneous
Lecture Lecture Lecture
Belittle Students
Teach What You Learned
Teach How You Learned
Impossible Exams
Trivial Exams
Waste Student Time
Teach Old Material First (Teach In Strictly Chronological Order)
Teach In Strictly Logical Order
Teach Easy Material First (and hard material last)
My Students Are Not Smart Enough
My Students Can't Do ___________(Whatever)
Teach Only Your Subject
Teach Only The Best
Teach Only The Worst
Assume Students Are Perfect
Assume Students Are Stupid
Assume Students Are Lazy
Assume Students Have Nothing To Do But This
Fill Up Their Empty Heads
Crap Shoot (grading seems random and unrelated to effort/knowledge/learning)
Never Any Exception To Any Rule
One Right Answer
Impossible Expectations
No Expectations
Ignore Student's Background
One Day Ahead Of Your Students
Accept Failure