Most people don't apply for their extension on the last day of their current permission to stay. If you are bounced back due to missing something, you leave and get that thing, and then try again. They don't deny you with a big stamp in your passport. They just refuse to accept your incomplete application. Most of the time you don't even make it past the document checker. There is nothing in your record and the immigration officers don't care. If for some reason it's something that you can't fix (for example a financial issue) that you can't fix, you either go to an agent and fix it with money, or you leave and return to start again. The 7-day thing only comes into play if you force the issue, and demand they accept your application. You will pay the 1900 baht for the extension, and they will give you an "extension denied, 7 days to leave Thailand" stamp. This stamp starts the day of your application, so in most cases you would probably get LESS time if you tried to do this, since most people would be applying with more than 7 days left on their stay. They won't just accept your application and money and give you a 7 day rejection stamp without telling you anything and it being a surprise. So the worst case scenario is you go see an agent, or pop out and back. Hardly the end of the world, and hardly anything to stress about. For 99% of people, they just leave and get what is missing and then return to complete the extension.
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