When you enter Mexico as a tourist you fill out an FMT (a Tourist Visa) form. The Immigration agent stamps it and gives you the bottom part of the form to carry as proof that you are legally in Mexico. When you leave Mexico the airline is required to collect that FMT from you before they ticket you. So if you've lost that FMT you have to go to the Immigration office at the airport to get a replacement. In the past Immigration has not charged for a replacement FMT, even though the law allows them to. But just recently Immigration has started charging for lost FMTs and this is causing HUGE problems. The cost is $525 MXP. You have to pay at the bank in the airport and bring proof of payment to the Immigration office there to get your replacement FMT. The cost is high, especially if more than one in your party loses their FMT. And it takes time to go to Immigration, then to the bank and then back to Immigration and then back to get your plane ticket. So people are missing their planes because of the extra time this is taking them. Be careful when you come to Cancun that you do not lose your FMT. It may not look important to you, but you can't leave the country without it!!