I miscalculated and was a day late to do my 90 day border run.

Fined 500 but also had to sign a yellow form printed in Thai but the blanks not filled out. There was also a special stamp in my passport, in which the officer wrote some things.

My guess is that the yellow form was some sort of confession/promise not to do it again/face the consequences if I do it again.

Anyone know what it was and the ramifications? (Australian, non-Imm-O)

Coincidentally, I have since had to renew my passport, so the next time I enter Thailand, the stamp will not be seen....but what of the yellow form?

I assume it is just an acknowledgement that you understood what you were being fined for. I assume you also got a receipt for the one day overstay.

The stamp was just the overstay stamp that shows the number of days you overstayed and the amount of the fine paid.

A one day overstay is nothing to worry about. Others have had much longer ones and have not had a problem.