A case study: a scheduling bug that stayed invisible until the clocks changed — and the test scenarios that would have caught it.
Every checkout test was green, but combining two discounts and a gift card drove the total negative — and issued credit. A case study in testing invariants, not just features.
Items vanished and duplicated on scroll; the frontend took the blame for two sprints. The cause was an unstable API sort over a non-unique key. Follow the symptom down the stack.
A negative quantity on a "remove stock" action inflated inventory and caused overselling. The feature worked; the absent negative test did not. Test the negative space at the endpoint.
A custom dropdown worked for everyone who tested it — because everyone used a mouse. Keyboard users hit a dead end on a required field. The cheapest check would have caught it.
On office Wi-Fi the payment flow was flawless; on cellular it double-charged. A client timeout shorter than real latency plus no idempotency, hidden by never testing a slow network.
A green suite, a flawless demo, and a permission check enforced in the UI but not the API — exposing every customer's data. Caught the day before release.
A single intermittent test trained a team to re-run, then ignore, then merge through every red build — until two real regressions slipped through.
Two days of regression, signed off green, and a regression shipped in the most-used feature. The tests weren't wrong — they were testing a stale picture.
Cut a dreaded two-day regression to an afternoon — and caught more — by weighting on risk, automating the stable core, and pruning dead cases.