Forgetting to Check the Airline Luggage Guidelines
I thought I hit the jackpot when I booked a ticket from Barcelona to Lisbon for only £30, until I arrived at the airport to find out my carry-on suitcase was far larger than the airline's regulation size. Budget airlines are unforgiving with ancillary fees, and suddenly my cheap ticket cost me over £100. Be sure to double check the luggage guidelines beforehand, because if you need to check your bag, it's usually much cheaper to prepay online before you arrive at the airport.