How's this scenario for you?
You arrive by plane Jan 1 and get a 30 day stamp. (you have a return flight flight Jan 29)
Jan 4 you decide to pop over to Cambobia overland to visit some obscure Khymer things.
Jan 8 you pop back to Thailand and get stamped in for 15 days, or until Jan 23 and welcome to overstay city!
The rocket scientists at immigration deserve a medal for this cleverness.
This will certainly help tourism in Thailand.