I'm An Illegal Alien
Really, technically I am an illegal alien in Mexico. When you go beyond the border towns of Mexico you are supposed to have a tourist visa of sorts that can be purchased the airports, etc. It's usually good for 6 months. Mine expired. Oops.
I don't expect I'll get in any trouble upon my return though. They don't actually look at them beyond just seeing if you have the paper. Even so, they're not exactly strict about it. I do need to get a new one next time though.