From a Slashdot post:

One professor of mathematics noticed that his kitchen sink at his home broke down. He called a plumber. The plumber came on the next day, sealed a few screws and everything was working as before. The professor was delighted. However, when the plumber gave him the bill a minute later, he was shocked. "This is one third of my monthly salary!" he yelled. Well, he paid and then the plumber said to him: "I understand your position as a professor. Why don't you come to our company and apply for a plumber position? You will earn three times as much as a professor. But remember, when you apply tell them that you completed only the seventh grade. They don't like educated people."

So it happened. The professor got a plumber job and his life significantly improved. He just had to seal a screw or two occasionally, and his salary went up significantly. One day, the board of the plumbing company decided that every plumber has to go to evening classes to complete the eighth grade. So, our professor had to go there too. It just happened that the first class was math. The evening teacher, to check student's knowledge, asked for a formula for the area of the circle. The person who was asked was the professor. He jumped to the board, and then he realized that he forgot the formula.

He started to reason it and soon filled the board with integrals, differentials and other advanced formulas to conclude the result that he had forgotten. As a result he got "negative pi times r squared." He didn't like the negative, so he started all over again. He got the negative sign again. No matter how many times he tried, he always got a negative. He was frustrated. He looked a bit scared at the class and saw all the plumbers whisper: "Switch the limits of the integral!!"