A promising PhD candidate was presenting his thesis at his final examination.

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!"