Beyond all edge cases mentioned by others (I add the bicycle frame size to the list) what is the most shocking for me having imperical units in international air traffic! Feets and pounds and gallons (as far as I know) mixed with SI units. One notable accident is the Gimli Glider where miscommunication occurred in fuel quantity calculations and the airplane took off with very little fuel running out mid air (excellent pilots and great story eventually).

Also naval folks are stubborn too. Own miles, knots as speed, craaazy! : )