I've always enjoyed one particular little feature of Mac OS X more than others: the Spotlight dropdown you can get by pressing the magnifying glass in the upper-right, or pressing cmd+Space. Granted, I mostly use it to launch programs, but it can also act like a calculator, which is a nice touch.
However, I started having problems with it in OS X Mavericks, which was for me a new version. For some reason, it wouldn't even do simple arithmetic like 3.6 / 12. Had the calculator in Spotlight stopped working? I did some quick Google searches, but nothing relevant jumped at me. What was extra weird was that only some calculations didn't work. Others did.
After an embarrassingly long session of troubleshooting, I figured out the problem: The calculator in Spotlight respects localisation, which in my case means it will refuse to calculate 3.6 / 12, but it will happily tell me that 3,6 / 12 = 0,3. It is so because at some point someone decided that in Sweden, we use the comma symbol as our radix point.*