In the summer, I wrote complicated code to test a client's eshop (the test creates a virtual user, clicks on buttons, makes a purchase, etc.). The client was satisfied, paid on time, no issues whatsoever.
Five months later:
Client: "All the tests are broken. Nothing completes, most of them get stuck. I need to get this fixed by tomorrow evening!"
Turns out, the client wanted more features, so they hired someone else to create a whole new eshop with the requested features, and this resulted in different texts on buttons, checkout process, everything.
Me: "Well, you changed everything, it's gonna take me at least a day or two to get to know the system before I can start designing tests, which will take another week."
Client: "Oh, well, I guess I'll just have to wait. But it's still going to be free, right? It's been five months since you delivered, so it's still a warranty fix."