Don't overstay a visa in Thailand. It's as simple as that.
The overstay rules are getting stricter and stricter.
At best you will have to pay a fine depending on the number of days you have overstayed. At worst (if the overstay is a long one) you will be barred from entering the country for X number of years.
There's little point going into detail. DON'T OVERSTAY A THAI VISA.