What happens if you get denied a visa extension due to overstaying in Thailand?

If you get denied a visa extension in Thailand after overstaying, you typically receive an "extension not approved" stamp, allowing you 7 days to leave the country. For short overstays, you can still apply for an extension after paying a fine of 500 baht for each day overstayed, plus a fee of 1900 baht for the extension application. It's important not to overstay and then seek an extension, as denial may lead to being required to leave the country immediately.