United States Marine Corps.
Currently operating as their own branch of the US Military, the USMC was originally infantry for the US Navy. What is not commonly known (even among Marines) is that to this day the USMC is still officially part of
the Navy. While they may operate independently of the Navy, the USMC has never technically been made its own branch.
Generally, Marines tend to despise the Navy (as well as the Army and Air Force) and its members, regarding them as inferior. This being the case, pointing out that the USMC is part of the Navy is often the simplest way to anger a Marine.
Angering a Marine can be very funny or very stupid. Do so at your own risk.