The classic was the Mars probe that crashed because the European part was giving its altitude in metres, and the American part was interpreting it as feet. Oops. In commercial software, we commonly put the units into the variable name, so instead of a variable called ‘height’, it would be ‘height_ft’ or ‘height_m’. Saves a hellava lot of problems.
I lived in Ireland for a few years, and while everything was officially metric, everyone talked imperial. Confusing. Drive over the border into Norther Ireland, and it was all imperial. I got caught behind a tourist (I assume) one day who, when crossing the border and seeing a ‘60’ sign on the highway, immediately slowed down to 60kph… 🙄