In our Fields and Waves course our professor had written the textbook, but it wasn't published in time for the start of classes so we were given a paperback "proof" copy. The professor offered us extra credit if we found any errors in the proof. When we were assigned a new chapter to study, the following day, he would ask if we found any errors. One of the chapters was so badly mangled that we could not understand it after re-reading it several times. One of the students said that he could not understand certain paragraphs. The professor read the beginning of the chapter. He then said to us, "When I wrote this only God and I knew what it meant; now only God knows!" Footnote to the story: We completed the entire course using the proof copy, but since we had paid the full purchase price for the text, we were able to get the final version after the course was over.
I also remember our Calculus professor who gave us what seemed like a really difficult problem. It turned out to be an April Fools joke. If you tried to use integral calculus to solve the problem, it would take forever, but in actuality it was a simple Rate x Time = Distance problem.