I'd argue that a suite of tests that exercise all reasonably likely scenarios is table stakes. And would have caught this particular bug.

I'm not talking about 100% branch coverage, but 100% coverage of all happy paths and all unhappy paths that a user might reasonably bump into.

OK maybe not 100% of the scenarios the entire system expresses, but pretty darn close for the business critical flows (signups, orders, checkouts, whatever).

Sure, hindsight is 20/20, but a bunch of these comments are replying to the assertion "And I can't think of one non-contrived reason to do it" (have a single test case with multiple subscriptions). That's the assertion I think is totally weird - I can think of tons of non-contrived reasons to have multiple subscriptions in a single test case.

I wouldn't pillory someone if they left out a test case like this, but neither would I assert that a test case like this is for some reason unthinkable or some outlandish edge case.

Go on then - so far the examples I've seen don't make sense in the context of stripe.