FMM’s get lost, are not stamped the right date or get overstayed. Let me shed some light on the matter. Don’t fret, for every problem, there is a solution, even in a country as bureaucratic as Mexico.
I have lost my FMM!
When you notice you have lost your FMM, you have to visit a local immigration office either at an airport or in town. You have to apply for a new one. The fee for that will be around US$ 40. You have to fill out a form, go to a bank and make a payment (make sure you get your name right on the payment slip!)
I left Mexico by land and forgot to hand in my FMM
If this happens and you plan on revisiting Mexico in the near future. Please contact the Mexican Embassy or Consulate. Most of the time it will not matter, but officially you have not left the country. So they might want to unregister you.
I forgot to check the date on my FMM now I overstayed
When this happens you have to visit an immigration office before departure. This can be done at an immigration office at the airport, or at the border. You have to pay a fine for overstaying. this fine will not exceed 6000 Pesos and will be calculated per day.
The replacement or overstay procedure of your FMM will take some time! Make sure you will not miss your flight!
After how much time can I re-enter Mexico using the FMM?
The immigration law does not state a certain time you have to spend outside of Mexico before re-entering the country. Technically you should be able to travel back within the same day, or even an hour. But you are registered in the system. I have heard of people being refused at the border after leaving and entering several times, or even entering for the second time. Changing border crossings will not influence that. Either way, you will be urged to apply for temporary resident status.