This, and any code base of significant size is unknowable and starts to produce bugs naturally. Requiring tests and verifying minimum coverage are a few things you can do to control the death spiral.
I have worked on large systems devoid of tests. Not recommended. I literally witnessed multi-million dollar losses that would have been prevented by requiring tests.
I have worked on large systems devoid of tests. Not recommended. I literally witnessed multi-million dollar losses that would have been prevented by requiring tests.