One of my profs made the following remark in the class:

“I hate late comers. They think that they can come a few minutes late and still follow the class. But then by induction they can skip the whole class.”

Clearly his reasoning has a mistake. The induction proof wont go through that ‘N’ for which he erases the black board !

My analysis professor mentioned the following comment made by Rudin : The whole of analysis is all about finding out when two operations commute ! (I could not find the source)

If one goes through the whole of Rudin’s books on Analysis – Principles of mathematical analysis and Real and Complex analysis – one finds only only two figures: The McGraw Hill logo on the front cover and the TMH logo on the back cover ! (Well Apostols book, for example, has more figures)
Mentioning to the fact that a random walk in one or two dimensions is recurrent but a three dimensional random walk is not, the great Kakutani made the following remark :
“A drunk man will find his way home, but a drunk bird may get lost forever.”