Windows’ versions went from 8 to 10, skipping 9. This was because some code existed that assumed that if the version contained a '9', then it was either 95 or 98. They assumed nonexistence of other versions containing '9'.

An API that exposes ParseErrA, ParseErrB, DataErrA, DataErrB. You’ve caught an error and want to classify it as either a parse or data error. You do this by checking e instanceof ParseErrorA || e instanceof ParseErrB. This is broken! It assumes nonexistence of ParseErrorC.

The JS code "number string".includes(typeof x) assumes nonexistence of a type called, say, "r str".