This is the third year I have received a 1 yr. multiple entry Non O in America prior to coming to Thailand. The first two years immigration in BKK stamped 90 days on both my passport and departure card upon entry. This year I entered Thailand in CNX and the date stamped on both passport and departure card is 1 year from the date of entry. I even questioned the immigration officer and he smiled and said yes. I am assuming that he made a mistake and that I must exit the country and reenter prior to 90 days. Not sure what to do as the date stamped is clearly one year.

Non immigrant O gives 90 days per entry.

Non immigrant O-A gives 1 year per entry.

Which do you have?

Non 0. It is not retirement. Purpose is to care for Thai friend with cancer. The officer even wrote on top of the departure card Non 0 and then circled it.

You need to go to immigration and get your stamp corrected.

I did not have to show proof of anything. I however did write a letter to the consulate explaining the circumstances. Just wondering what happens if I didn't do a border run and left in late April as planned. It is clearly one year.

If you try to use the one year there is good chance they would charge you for an overstay. They consider it your responsibility to check that your stamps are correct.

Instead of trip to CM Immigration, couldn't I just do a border run prior to my first 90 days? I would imagine it would get corrected upon reentry - yes?

You can do a border run just before your 90 days is over and return and get a 90 entry.