What are the consequences of being outside Thailand after your visa extension expires?

If you are not back in Thailand to apply for your next extension when your current extension ends, then you lose that extension and will have to start over with a new visa.

But if you are on the following extensions, you need to cancel them at immigration. Even if they just end, they still need to be cancelled, and you can be fined 500 baht per day up to a maximum of 20,000 baht for failing to cancel them:

1) Education extension

2) Work extension

3) Volunteer extension

That is because each of these are based on a specific activity you are doing in Thailand, and part of your application for these extensions you sign a statement saying you will notify immigration if your circumstances change. Therefore if you stop studying, stop working, or stop volunteering, you are required to notify immigration. Even if you just leave Thailand after you finished, you failed to uphold the commitment you made to immigration.