[rpedela]

If you don't actively maintain the tests, then yes you can get a wrong impression of correctness. But when done properly testing helps ensure a certain level of correctness and old bugs don't reappear. A good rule to follow is to create a test(s) for each bug and run the tests on every check-in which is pretty easy with continuous integration. This will mitigate most bugs before they reach production. And the ones that do reach production get fixed...forever.

[ams6110]

This "old bugs reappearing" how does this happen? People make the same logical mistakes in new code? Old buggy code is literally resurrected somehow?

[ygra]

Making changes again at the same place where a bug was fixed previously is a common offender. At least at my workplace we have regressions every now and then (and thanks to no test suite those sometimes make their way to the customer, yay). Basically every time you touch code you have the potential to break something. Even more so when a bugfix was done by adding code, not by changing it. People might accidentally rewrite the fix. If you have no tests that ensure that functionality stays that way it can be easy to break things in the process.

[rodgerd]

From my days as a release/build manager, the most common cause of this was people merging code from branches where fixes had been overlooked and similar horrors. The second highest would be people fixing something that "looked wrong" but was actually there to solve a problem.

[rpedela]

Yeah I should have probably used the technical term: regression. It is very easy to do. Sometimes it happens when code is rewritten/refactored. Sometimes a new feature utilizes a buggy code path that was not previously used/tested. Etc.

[cwzwarich]

It's pretty easy to make the same mistake that originally caused a bug when rewriting code.