Non Immigrant O visa....I have to go to a land crossing to activate another 3 months.....I will be one day overstay when I cross. Any problems besides the 500 baht fee?

In theory, there could be an issue if you are stopped and checked (at a roadblock) on the way to the border. I think this happened to a few people a number of years ago, but my experience is that Westerners are not checked at the roadblocks now. I have not heard of this kind of issue in recent years. At the border, there should be no problem.

I have been stopped at roadblocks before but they have not checked me Just the Asian looking people

Seems they give falang a pass

Will defer to the experts here, but it seems to me that a far more serious problem you may face is that the IO who stamps you back into Thailand may conclude, rightly or wrongly, that your non-O will be deemed to have lapsed as a result of the overstay,thus meaning that you would only be given a 30-day permission to stay max. Seems to me that the only surefire way of avoiding this risk would be for you to carry on over the border you're planning to cross to the nearest Thai embassy or consulate on the other side for a fresh non-O.

Will defer to the experts here, but it seems to me that a far more serious problem you may face is that the IO who stamps you back into Thailand may conclude, rightly or wrongly, that your non-O will be deemed to have lapsed as a result of the overstay,thus meaning that you would only be given a 30-day permission to stay max. Seems to me that the only surefire way of avoiding this risk would be for you to carry on over the border you're planning to cross to the nearest Thai embassy or consulate on the other side for a fresh non-O.

Visas are never cancelled because a person had an overstay. Immigration has no authority to do that.

Will defer to the experts here, but it seems to me that a far more serious problem you may face is that the IO who stamps you back into Thailand may conclude, rightly or wrongly, that your non-O will be deemed to have lapsed as a result of the overstay,thus meaning that you would only be given a 30-day permission to stay max. Seems to me that the only surefire way of avoiding this risk would be for you to carry on over the border you're planning to cross to the nearest Thai embassy or consulate on the other side for a fresh non-O.

Pretty sure that the overstay would only relate to your permission to stay granted by immigrations and not have any effect on a visa issued by an embassy ... assuming it hasn't expired.

Some border crossings, I believe, based on some reports, may send you to the local immigrations office to clear the overstay before allowing you to exit though. Can't find the posts about that, but think I have read of that in some thread(s).

In theory, could they not ban you for 5 years from entering Thailand because the overstay, regardless of how many days?

In theory, could they not ban you for 5 years from entering Thailand because the overstay, regardless of how many days?

NO !

There is no legal mechanism which permits anyone to be "banned" for overstaying ..............

In theory, could they not ban you for 5 years from entering Thailand because the overstay, regardless of how many days?

No

The only punishment is a max fine of 20k baht as written in the immigration act.

In theory, there could be an issue if you are stopped and checked (at a roadblock) on the way to the border. I think this happened to a few people a number of years ago, but my experience is that Westerners are not checked at the roadblocks now. I have not heard of this kind of issue in recent years. At the border, there should be no problem.

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In January 2015 I did a border run to activate the 2nd entry of my double entry tourist visa.

We went through Cambodia, and on the way to the border our van was stopped at a police/immigration check before the border. The van was full of mostly Asians, but there were three European Farangs (I was one).

We were all checked to make sure our current visa was valid by the immigration officer who was at that checkpoint with the Thai police that ran the checkpoint.

So, yes it can happen.