We were delayed at the border into Guatemala this past February when one of our viajeros couldn't produce the receipt for his tourist card.
No receipt for your tourist card, no Mexico exit stamp in your passport book.
No Mexico exit stamp in your passport book, no entrada into Guatemala.
He went back to the parking lot, swearing up a storm, looking for his tourist card receipt.
Meanwhile, a bus load of people looking to cross into Guatemala arrives, unloads, and lines up in front of the lone official with the precious stamp.
Our man eventually finds the receipt, and tries to go to the head of the line. No dice, the official is not having this. Head hung in despair, he gets in place the back of the now long line.