He wasn't a professor, but rather a middle school teacher I suffered....
After about months of working on an 8th grade science project... "How to calculate the depth of a crater on the Moon".... Instead of creating a model, I used diagrams and formulas drawn on the blackboard... The blackboard looked pretty cool with all the formulas I had created drawn all over it.
The presentation was to take about 25 minutes and the teacher didn't call on me till about 5 minutes before the bell.... he said he would listen to my report after class (it was the last class in the day). Ok fine.... well my classmates had all left leaving just the teacher, myself and the blackboard.... he just walked out about 5 minutes into my report without so much as a gesture, leaving me alone in the classroom. I just kept going and presented the remainder of my report to an roomful of empty desks. I finished my report, packed up my belongings and just went home. I did get an "A" on my report, but the incident probably kept me from a nice career in astrophysics. That is when I learned "Why should I care if no one else does?". I know I bore some people with long-winded technicalities and details, but really??
Much later in life, during university, I showed my old report to a Harvard credentialed physics professor just to see what he thought, He informed me that my formula was not totally accurate but was very creative and that I had inadvertently discovered a tangent using the Pythagorean Theorem and angles. Not too bad for a 12 year old. Something a decent, present and awake 8th grade science teacher would love to see in one of their students.
I hereby nominate my old 8th grade science teacher for "worst teacher of the 20 century".