I have a perfect real-world example. About four years ago some of my code broke in certain cases. I came up with a fix that relied on a case-sensitive regex t

I have a perfect real-world example. About four years ago some of my code broke in certain cases. I came up with a fix that relied on a case-sensitive regex to check for those cases. I think I made it case-sensitive because I wanted to make sure it didn't trigger accidentally on something added in the future. And these case names had never changed, right? Yep, now that I've spelled it out, what happened is obvious. Three years later, I got ordered to change one letter in these case names from lower case to upper case. Of course I didn't remember that I'd used a case-sensitive test against the names three years before. And bam, the bug was back, and as there was no test for it, I shipped code with the bug. The good news is the bug was obvious as soon as the customers tried to compile my code, so it didn't cause any harm but embarrassment on my part. Even so, it took me a while to track down what was going on. Imagine my shock when I got into the code and found the fix I thought I needed to make was already there... but itself needed to be fixed!