A few months ago I bought flights on Alitalia. I paid for seats and got emailed confirmation. When I did OLCI on my phone, the seats had changed so I tried to move us to our original seats, but payment was demanded. I tried thinking I'd sort it at the airport. It wouldn't accept payment, so I abandoned the transaction and checked in on my PC. I printed BP's with the original seats and when I was about to board at FCO, the people in front had the same seats, so they moved us to the back of the plane, despite evidence that I had paid in advance.

I have complained to Alitalia. Is this sort of practice common?

The aiport staff were most unhelpful.