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.