So, there I was with my new non imm O multi and I get stamped in until the expiry date of the visa: Oct 2010. I always have a check when leaving the desk so went back and after explaining to the officer that I think I should only be entitled to 90 days he has a word with his neighbouring officer who agrees and stamps me in again.

I leave the desk again to go to baggage claim and check again and he has only stamped me in for 2 months until Dec. Back I go and he disagrees showing me his fingers 1 = oct, 2 = nov, 3 = dec.

He has a word with his neigbour again when I say it should be Jan and finally after they have a discussion, I am admitted until January. All with smiles and good humour and lots of "geng" (clever) directed towards me but come on, I didn't know whether to laugh or cry at the pair.

So my question is what would have happened if I had just the left the original stamp of 1 year?

You would be on overstay after 90 days. Even if immigration fouls up it is your responsibility to make sure you do not overstay the proper entitlement of stay and to get your stamp corrected. Great system

You would have been overstay for 9 months and most likely fined 20,000 baht if it were not found out earlier. You are responsible for making sure stamps are correct; as you did.

After 90 days you would have been on overstay.

Nothing much would have happened, as long as you would have left Thailand within 90 days of entering!

Immigration in principle doesn't care what is stamped in your passport, it cares about how long you are legally allowed to stay on the visa you utilized when entering.

Then again, when suspecting a wrong stamp, it would be prudent to go to the main Bkk immigration office (now in Chaeng Wattana) where I assume they will have a desk dedicated to correcting wrong stamps (they had one at the old office)...

Or obviously the best way is do as you did, check and get it corrected on the spot!

It's never their fault !

Reminds of the Post War Post Office Savings Book in UK

On the back cover it stated

Government Property Its loss wil cause YOU trouble

Organise up brewery piss in couldn't an

.....................................................................

Arrange-Re to in a known well saying or phrase

If someone were really clueless (or devious), and decided to stay longer than 90 days in this situation, how would they be 'found out' by Immigration at the airport when they eventually departed? Or would they?

I've never seen Immigration cross check the 'permitted to stay' stamp with the source visa/re-entry permit -- a waste of time, since 99% of the time this would check-out 'ok.' Would something in a database sound an alert?

Just curious (As obviously this is not a frequent situation, open to exploitation.)

Every person leaving Thailand is checked to some extent and a one year stamp would likely call for more than normal checking to be sure the person was not leaving without a re entry permit and try to save him from losing his visa/extension of stay.