I guess I'm not sure why so many are so eager to avoid the rules. If you're not sure what the rule is, go by what the book says, you can never go wrong. Someone may have had a different experience, but as time has shown us here on the forum, it will someday bite someone in the behind.
Then, as my father would say, you come crying saying how unfair the rules are and you'll never come to Mexico again.
This is the same dilemma con Mexicans that have a permit for travel to the U.S. Even though it is good for six months, the law says you have to return it to the U.S. customs, in person, at their office at the border. Well, sometimes they check and sometimes they don't. But when they do, it's boo hoo hoo. Why doesn't the rule apply to everyone? Well, we're human beings and some to their jobs well and other don't. Some say don't worry about it but they won't be there to hold your hand when something goes wrong.
Same case here last month on a trip to Pennsylvania. As a permanent resident I have to check out. I told the guy I may not come back by air and asked if I could drop it off at the aduana in Reynosa. He said that the bus didn't stop there. I asked him what I should do then and he said, "don't worry about it".
Well, I'm much smarter at 60 then I was at 30. When I got back, I drove the 150 kms roundtrip from my house to the airport to turn in the form and have my passport stamped. May sound silly to some but I sleep like a baby.
