I have double booked the tickets for Cirque du Soleil by accident.
(so 4 tickets instead of 2)
The website gave an "error" message while doing the payment with nothing else happening and no confirmation coming in in my inbox.
So, I booked the tickets again and I received two emails after the second payment attempt.
The tickets are not cheap so I'm hoping that I can cancel two tickets. Keep everything crossed for me, I'll be phoning them first thing in the morning.