A couple days ago I was booking the ferry for our summer vacation. There is a bunch of shipping companies out there, for example "Corsica Ferries", "Adria Ferries", "Condor Ferries" and so on. I was though booking via directferries, which is an intermediate platform, like a middleware. There are other platforms like that in the airline industry, for example ebookers.
I was finishing the transaction and at the end I got the following error message, which says something like: "We have received the booking request, but haven't received the confirmation by the shipping company. Please contact us by phone to confirm the booking. You credit cart WAS charged."
Not very comforting, and my car can definitely not swim, so I need that ferry. Well, the next day somebody from directyferries called me and we walked through it. We figured out that the problem was fairly simple: My last name is "Schär" (see the two dots over the a). Well, directferries would accept them, but it would screw up the booking system of the shipping company.
Teasting is not an easy thing. Probably they did test the platform, but probably only with names like "Peter Muster". The testers have not been creative enough. Maybe the testers were located somewhere in India and don't even have the "funny" letters like äöü on their keyboards.
When you test: Make sure you have enough time and try to think what the users might possibly do. Who are your users, where do they come from, what could they try to do to the system in order to achieve their goal. Be creative!