I had a similar experience having Astra add TLA+ and Lean verification tests to a project with somewhat complex state and many small algorithms (almost all generated by coding agents). The system had been working quite well, but adding formal verification surfaced 33 classes of bugs, some of which had several instances.

I completely agree with you about how fun these things can be. I've long believed in automated testing methods that were beyond what I could justify. I learned about formal methods in college in the 1980s and have never been able to justify (even to myself) applying them. Now a volunteer project I've been working on has 100% standard test coverage, many property-based tests, and 1000s of formal verification tests. That combination has surfaced multiple bugs in widely used underlying libraries and a bug in Rosetta 2's Intel emulation that was affecting me. And that's all just on the testing front. I'm having a lot of fun with all this.