I ran into this with a flight reservation system. I lost hundreds of dollars and missed my flight when the front end accepted my email address but the back end

I ran into this with a flight reservation system. I lost hundreds of dollars and missed my flight when the front end accepted my email address but the back end didn't send me a confirmation. A plausible explanation given to me was that the web application worked just fine, but it sent the data to some COBOL-ish back end written long before the advent of web bookings, and the integration code mangled my email address when it stuffed it into some data field that was never intended to hold an email address. This is why any real-world system needs end-to-end integration testing and not just unit testing :-)