Formal methods are often classified as irrelevant and/or impracticable by practitioners. This is not true: we found and fixed a bug in a piece of software that is used by billions of users every single day. Finding and fixing this bug without a formal analysis and the help of a verification tool is next to impossible, as our analysis showed. It has been around for years in a core library routine of Java and Python. Earlier occurrences of the underlying bug were supposedly fixed, but actually only made its occurrence less likely.