OVERSTAY Warning
The topic comes up enough I thought I would touch on it.
Overstaying your permission to be in thailand is NEVER good and it is totally against the law.
It is not necessarily the overstay itself that puts you at risk because if you turn yourself in (as in stamp out of the country) you can't get banned for overstay until you have overstayed 90 days or more.
What is dangerous is being on overstay and getting caught out for something else by the police. That is when you can come right off the rails. If you're arrested even on an overstay of a single day you can be deported and banned for 5 years.
A single days overstay fine is usually waived at the airport when flying out, although if you exit by land you pay for every day overstay.
I have seen people who were going to be on an overstay of 3 days (1500baht fine) go to the immigration office and try to buy that 7 day extension denied stamp so they were legal only to be denied the stamp and told by the immigration officers to just pay the 3 day overstay fine at the airport when they left.
At this time, an overstay stamp in your passport does NOT affect your ability to get a visa for thailand from a thai consulate, and does not affect your ability to enter the country.
I would say IF you are going to have a short overstay, keep your head down, don't get caught for something else, get to the airport leaving plenty of time to pay your fine and get your overstay stamp. It's not the end of the world. Hundreds, perhaps thousands of people overstay every month here.
Good Luck (y)