Especially with duck-typing, you might also assume that a function that previously returned true-false will work if it now returns a String or nil. Semantically they’re similar, but String conveys more information (did something, here’s details vs did(n’t) do something).

But if someone is actually relying on literal true/false instead of truthiness, you now have a bug.

I say this as a Ruby evangelist and apologist, who deeply loves the language and who’s used it professionally and still uses it for virtually all of my personal projects.