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 advice
Walking past the Royal Courts of Justice one day, a man spotted a friend of his sitting on the steps outside, sobbing loudly with his head buried in his hands.

"What's the matter?"

he asked of his friend, "did your lawyer give you bad advice ..?"

"No - it's worse than that," replied the friend between sobs, " he sold it to me..."