Overstaying your date in your passport is breaking the law.
Many times on here people ask is it ok to overstay - 1, 2, 5 days etc. The fine at the immigration exit desk is 500 baht per day of overstay. Some immigration officers will waive the fine if it is only 1 day, but you’ll still get a red stamp in your passport.
HoweVer if you are stopped inside the country for any reason - like at a police check point - even on your way to airport or border crossing then be prepared for a whole lot of trouble.
The police may arrest you as you have broken the law. If they do you’ll be fined in court and then sent to the nearest Immigration Detention Centre, where you will be kept until you buy a one way ticket back to your home country. If you don’t have the funds for that you will be kept there until you find some means of raising those funds. Once you have your ticket, you will be deported and blacklisted from entering the country for up to 5 years.
Sounds dramatic but it sometimes happens. 500 baht a day sounds cheaper than 1900 baht at a local immigration office but in the overall scheme of things it is never wise to overstay.