Defects that arrive every month until somebody notices.
A coupon that should have expired after three months does not. Every invoice looks correct. It is wrong by a fixed percentage, monthly, forever.
Anyone running subscriptions — SaaS, membership, or anything that charges on a schedule.
No charge at any point, conversion on exactly the stated day, and the abuse paths most implementations miss.
A period charged exactly once — and the month-end anchor problem that walks a customer's billing day backwards.
Recompute one invoice by hand. It finds more billing defects than any other single technique.
The retry schedule, and the notification column where the recoverable revenue actually is.
The defect most likely to end up on a public forum with a screenshot.
Money taken against money owed, checked continuously rather than at the end.
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.
Every invoice is internally consistent, correctly formatted, and wrong by a fixed percentage — applied to every affected customer, every month, until somebody notices.
Severity here is a rate, not an event.
A subscription anchored on the 31st has no valid date in February. Clamp permanently and the customer's billing day migrates to the 28th and stays there, quietly shortening every cycle.
Only visible if you advance the clock through a short month.
The first renewal after cancellation is correctly skipped. The second is not. Ninety days of testing finds it; thirty does not.
This is the one that ends up on a public forum with a screenshot.
One download. The playbook, the case bank as a spreadsheet, and the practice sandbox. No subscription, no account.
Buy — $39International · card, PayPal কিনুন — ৳2,400Bangladesh · 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.