I have been spending a fair bit of time reading American websites and knitting patterns, which refer to everything in terms of inches, rather than centimetres. So I am used to automatically multiplying and dividing everything by 2.5 as I switch between American and Australian measurements. (1 inch = ~2.5cm)
I needed to buy a 2mm circular needle for knitting socks and through experience have found the 100cm-length needles are best for the Magic Loop technique. So do I buy a 100cm-length 2mm needle? No, I absent-mindedly automatically divide 100cm by 2.5 and buy a 40cm-length 2mm needle instead. And don't notice until I am home, miles from the store.
American cultural imperialism strikes again!