[Too]

In isolation, bugs don't suddenly reappear no. So if you only regression test on unit-level, it's very unlikely the test will ever fire red again. What you should do is write use-case regression tests. This will protect you from major regressions, such as "user-registration is taking several minutes", because this is a major bug that can occur for a million new reasons, even if last time the reason was isolated to "database query x was running very slow", your test should track the former, not the latter.