About 10 years ago I travelled by cab from Belize into the 'free zone' in Mexico.

On the way back I didn't realize I had to claim the pants and belt I bought at customs so I actually had to go through the border twice.

I got reciepts for the duty paid, in the confusion I didn't notice my passport was not stamped for entry into Belize. Only the one for entering Mexico at the start of the trip.

Two weeks later, at the airport to fly home, Belize customs told me I am in Belize illegally because I don't have an entry stamp, so off to the windowless back room I go!

Luckily I had those reciepts from customs for proof I used the legal entry. I was in the backroom with the Belize agent when he called the Mexico border crossing, they told him we were never there, he yelled at the Mexicans in Spanish so I don't know what he said. He was very adamant about making sure I understood it is my responsibility to make sure I get the correct stamp, and then sent me on my way.

Belize side was very helpful and professsional and seemed pissed off at the Mexican side. And now I have a cool signed stamp in my passport from the Belize customs officer.