Help,
Well I did it now. I booked my flights, paid for the rooms and now working on the insurance and found that my stay is for 91 days. Is there a way to get that extra day on visa and not be fined? We fly back to the airport on day 90 and fly out on day 91. Which looks like it would not be insured plus it is a one day over stay. What a mess!
TLDR : Answer Summary
The user is concerned about a one-day overstay in Thailand due to planned return flights. Commenters confirm that typically there is no fine for a single-day overstay when departing by air, though there may be a stamp indicating the overstay. Recommendations include booking a late flight to ensure departure on the visa expiration day.