Of course I still recall "cooking" a pork roast for friends one sunday and totally messing up the conversion from kgs to lbs and way WAY undercooking the pork. Porl tartar anyone?
Then there is the famous
Gimli Glider where a 767 was under fuelled due to a mixup in conversion between metric and imperial units, and was glided to a landing on a former RCAF airbase where folks were drag racing.