On: A routine gem update ended up creating $73k worth ...

[nightpool]

This is a deeply fundamental library that's used across their entire app. How likely would the author have been to find this subtle of a bug in such a minor part of their application code, if their app was of any appreciable size?

Depending on how their testing environment was set up, they might not even have enough database users in their DB during automated testing to notice this bug if it did trigger.

[mdavidn → nightpool]

A method that bills customers is not a “minor part” of the application. This is where automated integration tests can make the most difference.

“It should not call the Stripe API if customer has credits remaining.”