I did it deliberately a few times over the years, I never had any repercussions for the tiny one day overstay stamps with a squiggle in my passport. This is my 20th winter in Thailand.
As I thought, you are referring to retirement extensions and not retirement visas which generally must be obtained outside of Thailand. And these extensions are not multiple entry but a one year multiple re-entry permit can be purchased for 3800 THB.
That sounds more reasonable, but mine also wanted a witness to verify l was staying there. But they were out of luck there because there was only one other Thai resident in another apartment and he always went home to his family at the weekend. So they had to be content with my girlfriend as a witness.