[jbattle]

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).