I've made this mistake myself, and its a price you have to pay to have the convenience of being able to book your own tickets. There are several confirmations you make before you pay, so you likely would have been able to catch it previously. I've also had people get confused on tickets that are departing just after midnight on flights, and miss their flight, thinking it departs the following night, and the airlines still generally make you pay. Its a lose-lose situation! I have also had itineraries get messed up by having multiple windows open for the same website while researching fares and have the date incorrectly get booked because of that, realizing moments later when the confirmation email arrived. They checked their database and sent me the log of my transaction and showed me that I had in fact chosen those dates, and made me pay the change fee. If you truly believe there was a technical error, you can contact the airline and they will be able to have their tech support/web team look at the transaction for you and see if it was in fact their error. Most likely, it was yours and you'll just have to pay unfortunately!