A lecture about English
A linguistics professor was lecturing to his English class one day. "In English," he said, "A double negative forms a positive. In some languages, though, such as Russian, a double negative is still a negative. However, there is no language wherein a double positive can form a negative."

A voice from the back of the room piped up, "Yeah, right."

Bad temper problem
Patient: Doctor, you must help me. I'm under such a lot of stress, I keep losing my temper with people.

Doctor: Tell me about your problem.

Patient: I just did, didn't I, you stupid fool!!