Famous or semi-famous cases of math professors making errors

Question

I don't know if this question is best suited to this stack exchange. If it isn't, feel free to migrate it or close it. This question was inspired by a mistake I saw in a math class. I corrected the professor, and he acknowledged it. I then said, "Some students think professors never make mistakes". And he said, "Yes, and those students are mistaken". So, what are some famous or at least semi-famous examples of math professors making errors in the classroom? Note, the mistake has to have taken place in a classroom, not in a journal or book or paper.

Answer 1

Grothendieck is famous for having chosen $57$ as a prime number example, when answering a question from a student.

And actually the same mistake had been made before, by Weyl, in a paper, as can be read here. https://hsm.stackexchange.com/questions/6358/story-of-grothendiecks-prime-number

Answer 2

Aristotle famously taught that the only regular solids that tiled space were the cube and the tetrahedron when, in fact, only the cube does so.

It took 1800 years for the mistake to be corrected.

Mysteries in Packing Regular Tetrahedra

Note: It is generally accepted that most of Aristotle's surving writings were, in fact, lecture notes. So the tetrahedron error is an error which he would have made in a class lecture.

Answer 3

Wiles initial proof of Fermat's Last Theorem contained an error, and in some documentary he said he was about to give up on it, but tried to understand one last time what the exact problem was, and then was able to fix it (he also had the help of Richard Taylor his previous graduate student)