I just "tried" buying tickets on RyanAir from Wroclaw, Poland to Nottingham, England. I couldn't get their system to accept my credit card. The response was that the credit card number or address was incorrect. Wrong!
What really irks me is that I couldn't find a number to call and talk to a human. That frosts me. I know you keep costs low buy automating but when there's a problem you need someone to talk to. Especially if that problem is not yours.
I'm grumpy right now.