I use www.ba.com to book Heathrow-Oslo flights a lot. I just booked another one and accidentally specified a return day of March 20th instead of February 20th. I'm pretty careful about this kind of thing but this time I got caught out, partly because both days are a Saturday. I spotted it immediately and changed it straight away. For a flight costing £265.50 ba charged me an extra £200 which frankly I find outrageous. Their website says to ring 0844 493 0787 for help. I tried that - it's a waste of time. Endless recorded messages saying they're sorry lots of passengers have been separated from the luggage because of the snow. But no human being.
I'm reminded of something Jeff Bezos said recently - that there were two kinds of companies; those that work to charge you more, and those that work to charge you less. All my previous Heathrow-Oslo flights have been for one week. Suddenly their system was being asked for a much longer stay. It surely wouldn't take a lot of effort to get the system to spot that sudden difference and before accepting the booking simply say "we've noticed your booking is for a longer period than usual - are you sure it's correct". If their system had done that I would have been very impressed. I would have told my friends. But it didn't. Instead it charged me an extra £200.