[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.