BTIZ wrote:

If your
altimeter setting is not set to "local", his computer radar will
misinterpret the altitude ModeC information from your aircraft.

It won't in most aircraft. The mode-C encoder used in small aircraft has it's own
little altimeter, and it's not affected by the altimeter setting you're using.
When the controller tells you the altimeter setting, he or she is telling you
that you are flying at the wrong altitude. They're going to give you the benefit
of the doubt and assume that you just set the altimeter incorrectly, but they
know it's really because you're a klutz.

George Patterson
Battle, n; A method of untying with the teeth a political knot that would
not yield to the tongue.