I'm not sure it's that subtle. In the screenshot it looks like a random user was charged each time the method was called. A test for "run the query, notice the user was charged" would have failed. Maybe they got very unlucky and the test randomly picked the correct user, or maybe they ran the test against a database which contained just a single user, but that last part is within their control.

> I don't think any reasonable level of testing would have been thorough enough to catch such a subtle issue.

If there was a staging environment which tried to match production as closely as possible, and end to end tests of this feature were run, then this bug would have been caught. That doesn't seem like an unreasonable level of care for something as sensitive as billing.