Re: Testing
Costs can be shocking if customers overload your systems/infrastructure or cause harmful other events, like toxic data or security compromises; ... like World Pay!
Testing should include some (sanitised) saved real data, because users and systems can do the damned unexpected stuff; been there! Also load testing to check for overload of systems/infrastructure or race condition failures, and input fuzzing, excessive input, and specific exploit testing, to check for inadequate input validation and for security issues.