You'll receive an overstay stamp in your passport. But at airports the fine for one day is waived.

You're not unlucky. You've outstayed your welcome as a tourist.

You're lucky you were not denied entry. Immigration no longer considers you a tourist as you're clearly living in Thailand. You're all but guaranteed to be pulled out of line next time and very possibly denied entry for trying to live in Thailand without a long term visa. They're currently denying entry to hundreds of people per week for abusing the system. You can find many reports of this across the internet and even in this exact group.

And entering on a different passport is going to raise even more red flags as when you scan your fingerprints they're going to see your entire record and assume you're trying to hide something by using a different passport.

@Baz ********

that's what the residence certificate from immigration is for.

You should be able to get it from your immigration office after receiving the extension.

@Ian ******

being a foreigner

@Yo***

You will not be able to get a new long-term visa or extension from immigration (for example a new work extension) until you cancel the previous one. You would then receive the fine of 500 baht per day (to a maximum of 20,000 baht) for failing to cancel. Immigration doesn't care why you didn't do it, they just know that you did not as you agreed to.

@Hillary ****

stop giving bad information. We're not talking about overstay. We're talking about an education extension. You won't be fined at the airport. You'll be fined when you go to immigration in the future when they see you failed to meet the requirements of your extension. And an extension is not the same thing as a visa. We're talking about specific extensions here.

@Hillary ****

that will invalidate it if no re-entry permit was purchased. But that doesn't mean it was cancelled properly and is a failure to meet the requirements that were signed when getting the extension.

The non-O visa, which most people use for retirement, does not require any insurance.

It's only the non-OA that requires medical insurance, and you would not be able to use your own insurance.

Work extension, volunteer extension, education extension all have a provision that you must notify immigration if the basis of your extension changes. This means even if you just let it expire, you are in breach of the documents you signed that said you would let immigration know of any change in status. All of those extensions must be cancelled with paperwork from the sponsor or else you can be fined 500 baht per day for failing to abide by the agreement you signed (up to a maximum of 20,000 baht) and can also prevent you from getting a different long-term visa or extension if immigration or the embassy notice that you failed to cancel the previous extension.