I don't know if I'm confusing fact with fiction here, but I recently heard something about a student who used Eratosthenes' method of measuring the diameter of the Earth... and got an answer of 1.2 metres! If true, it sounds as if the student had the necessary maths skills, but not the commonsense to check for obvious errors.

And even if it's not true, it's sufficiently reminiscent of the Mars probe where they confused imperial and metric measurements. Or one time I was writing up a school lab experiment, and I said the equipment was delivering water at a rate of 40 litres per second. I wasn't working with a fire hose!