I hate legacy code.
Introduced some new changes to our application and voila! A bug in the legacy system surfaces. It was just hiding in there, waiting to ruin my weekend.

Sounds like an opportunity to introduce integration and regression testing

@SortOfTested I think it's worth starting a bit more locally -- hi-level unittesting, where units are features, e.g. controller methods in case of a webapp.

This way one would not need to waste time on deployment to find smth is not working