Comment by MyFedora

Don't worry, professors do it too.

I had one such professor in accounting. Typos, numbers from wrong year, copy/paste mistakes, forgot to turn on precision as displayed, mismatching account numbers, etc.

The textbook (written by someone else) also had mistakes. At least not in every exercise, but enough to be annoying.

Exams were also graded based on an incorrect solution, so you always had to fight for a revised grade.