I wrote some property-based tests for a parsing library. Eventually one of the tests for case-insensitive parsing failed... because it hit the letter "DŽ" which has upper, lower and title-case.

The funny thing is that the parsing library was correct and it was the test property that was wrong—but I still learned about an edge case I had never considered!

This has been a common pattern for "simpler" property-based tests I've written: I write a test, it fails right away, and it turns out that my code is fine, but my property was wrong. And this is almost always useful; if I write an incorrect property, it means that some assumption I had about my code is incorrect, so the exercise directly improves my conceptual model of whatever I'm doing.