OVERSTAYING IS ILLEGAL !!! >:(
I think we need to touch on this subject because topics about overstay come up at least once a day.
Plain and simple it is against the law to overstay, period, end of story.
The fine for overstay is 500 baht a day and caps out after 40 days at 20K baht.
The overstay/banning penalties have been in effect since March 20, 2016.
Too many people think it's no big deal and truth be told short overstays are usually no problem and at this time having an overstay stamp in your passport doesn't hurt you when you entering the country again.
The thai consulate in Penang recently started a policy where they will not issue a tourist visa to people who have an overstay stamp in their passport. There is no telling when/if other consulates will start following this policy.
My advice to anyone is DON'T overstay!