Testing shows the presence, not the absence of bugs.
Apparently, software regression is a very nasty situation in the development process. It usually means that the last delivery contains something breaking. To overcome the situation the whole release must be analyzed. A developer has to write tests, rollback the changes, run tests, and … it is still there, one more step back in the VCS history and the error is still reproducible. And now this bug just got another additional label “legacy”.
Actually, it turns out that this functionality has not been used for a while thus the bug could be introduced not just with the last commit or two but quite some time ago. In case if the codebase if big enough it may lead to some significant amount of time to find an exact change that introduced this bug.
In practice, there is a way how to automate this search. Below there is an example of this operation within the Git repository.