The edge cases that cost real money.
A customer says they paid. Your dashboard says they did. The gateway has no charge. Which one is lying?
Anyone shipping a checkout — e-commerce, SaaS billing, marketplace, or a local payment rail like bKash or Nagad.
Not an error — a normal outcome that needs designed behaviour. Cart preservation, stock release, and what the customer is actually told.
Challenge, completion, and the abandonment case that leaves an authorised-but-uncaptured charge behind.
Double-click, refresh, back button, network drop. The defect a real customer finds by accident on a slow connection.
Replay, ordering, signature verification. An unsigned webhook that marks an order paid is free goods.
Partial refunds ignoring discounts, tax computed before the discount, per-line versus per-order rounding.
Cart, order, charge, ledger. Four records that must agree — and the variance that appears when they do not.
A working application with 18 deliberate defects, each switchable on and off individually. Run a case, watch it fail, disarm the defect, run it again.
That second run is the point. Most testers have seen software fail. Very few have deliberately observed what a genuine pass looks like for a specific control — which is why juniors report defects that are not there.
Python, no installation beyond the interpreter.
The reasoning behind the cases. Why this domain fails differently, which checks matter most, how to report a finding so it gets fixed rather than filed, and the sign-off that turns a test run into a deliverable.
A filterable spreadsheet with preconditions, steps, expected result and severity for every case. Plus a bug report template, a client sign-off sheet, and:
The eighteen checks worth running before every release. Included in the kit, and free to anyone — take it without buying.
In full, so you can judge the rest. These are the kind that come from handling the incident rather than reading the documentation.
The customer opens the bank's page and closes the tab. No order exists, but the hold on their card does. They see money gone and no purchase, and they call support angry.
Nobody writes this case from a feature list. It comes from having handled the support ticket.
Intercept the checkout request, change the amount, submit. If it goes through, every product in that store is free to anyone who can open developer tools.
Sixty seconds to test. It has ended businesses.
Arithmetically self-consistent, correctly formatted, and wrong on every invoice by a fixed amount that scales with every customer you add.
The invoice looks right, which is why it survives review.
One download. The playbook, the case bank as a spreadsheet, and the practice sandbox. No subscription, no account.
Buy — $49International · card, PayPal কিনুন — ৳2,900Bangladesh · bKash, Nagad, cardThe live demo runs five of the checks against a simulated system, in your browser. Nothing to install and nothing to sign up for. Watch a defect fire, then switch it off and watch the same check pass.