A while ago I had a weird situation with Flixbus (German, international bus service). I tried booking a ticket using different Credit cards, PayPal etc, and while the bank would authorize and block the amount, the booking would fail and Flixbus would have nothing on their systems about the attempts (I checked with them).
One month later, I tried again and same problem. Funnily though, the same card worked on the first attempt when I stopped using a hotmail address and switched to a gmail one 
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The user gets no error message at all that could indicate the problem, and it’s crazy how many failed bookings they (Flixbus) might be completely unaware of. They use AWS btw.
Suela.