You get a bit numb to this when working on old code (10+ years), you know there's plenty crash bugs and logic bugs in the code and you know you will introduce a few of them while fixing bugs and adding new features.

The only thing that can give comfort is automated tests and plenty of them! Unit, integration and especially system level for old code.

Good companies priotize this, and can refactor code without breaking too much, other companies learn to live with the bugs breathing down their necks.

That's my secret, I never run the code. That's the testers' problem.