A promising PhD candidate was presenting his thesis at his final examination. He proceeded with a derivation and ended up with something like:

F = -MA

He was embarrassed, his supervising professor was embarrassed, and the rest of the committee was embarrassed. The student coughed nervously and said “I seem to have made a slight error back there somewhere.”

One of the mathematicians on the committee replied dryly, “Either that or an odd number of them!”