What happens if I forget to do the 90-day report on a Non-O retirement visa in Thailand?

Your 90-day reporting window in person at the immigration office is between 15 days before the due date and 7 days after the due date. You can submit a report within this timeframe without any issue. If you are late to this "7 days after the due date", you will be fined 2000 baht for not reporting.

All the penalty for lack of reporting is a 2000 baht fine. Failure to report doesn't affect the next application. However, you will receive a stamp mentioning a 2000 baht fine on your passport. Some people don't care about it at all, and some don't like to have it.

If you leave the country and come back (ie visit Laos) the 90 day clock resets and you can report as usual without a fine, in my experience.

Cough up the cash and don't forget next time

You can submit the report up to one week after the due date. This is usually not a major issue until the next time you deal with immigration. I believe the standard fee for a missed 90 day report is 2,000 baht.

Once you are late in filing the 90 day report...

The fine will be 2,000 Thai baht...

And only will be collected the next time you need the services of your local Thai Immigration office.

No need to do anything else now.