It's not the "over-staying" so much that puts you in a world of hurt here in the glorious "Land 'O Thais", it's getting caught inside the country for something else while on overstay.

I know many people who've overstayed (some for years and years), yet never had a problem because they weren't "caught out" by the authorities while inside the country. They just kept their heads down and went about their daily lives. When they were in the position to clear up their visa woes and the overstay fine, they went to the border, paid the fine, got a new Thai visa in a neighboring country and came right back in.

As has been explained; the fine for overstay is 500 baht a day, and that caps out at 20K baht. So overstay 40 days or 40 years and in theory the fine upon exiting is the same, 20K baht.

Once you're caught inside the country on overstay (assuming you can't "negotiate" a way around it by whoever caught you), that's usually when you're arrested, transferred to Immigrations Detention Center, see a judge, and then get deported.

Now remember, simply being deported doesn't mean that you can't turn right around and come back in the country. It just means you were "invited to leave" and in fact you're taken to the airport by Immigration Police. I don’t know a single person ever black listed for just an overstay. Most of the black listing happens because the person ran amuck with some other much more serious problem while they were also on overstay.

I've sent quite a few people on overstay to the border crossings and not once has anyone gotten "banged up". So my opinion is you've got no more chance of having a problem leaving Thailand by a land border than you do flying out; ONCE you get to the border. It's those random police checks on the way to the border that can cause you problems. Once you're at the border the Thai officials there couldn't care less if you're on overstay or not; they just fine you the appropriate amount, put a stamp in your passport that you overstayed and let you leave Thailand.

As far as the nonsense of always carrying your passport. I sometimes volunteer at the local Police Station (although it's not as much fun as you'd think); in a week I bet 100 people come in with the "My passport got stolen, lost, etc" story, to get a Police report so they can go to their embassy and get a new passport. Make a copy of the front page, and the current "permission to stay until" stamp, and just carry that. If you're stopped you can show the Police those and 99.99% of the time that's enough.

If you live here I'd suggest you get a Thai driver's license instead. They're cheap, easy to get and are "valid I/D" here too. Most of the times I've had Police stop me, showing the Thai driver's license lets them know you "ain't a tourist", and they usually wave you on your way...