Back when I was starting out in a full stack role, I worked on a fairly big chunk of functionality that would trigger off a few entry points. It was wonderful for a few months. As we approached go live, our QA team started reporting weird intermittent issues. The logic wouldn't "trigger" the first time, but would on subsequent saves. Worse yet, the state required resetting of data every time we needed to test. Three weeks later, it boiled down to a 2 second time difference between the database's GETDATE() values and the new Date() object we passed in from our application.

I'll never forget that one system should be the source of truth again.