Even with inadequate tests, inadequate monitoring, and an inadequate integration testing environment; how did it not go like this:

"Hmm the web server appears to be working, but maybe the database is making an oopsie we did just do that whole stupid db migration without a gradual ramp up"

"Oh hey here's an idea, why don't we make sure error messages from the database are logged maybe it's silently crying out in pain right now :D"