How do I extend a 60-day visa in Thailand if I have an overstay?

Went to Immigration 1 to extend my 60 day visa (Canada) While they were processing it they realized I had been stamped in for 61 days at airport(I did not calculate it either) but they made the correction there and did give the extension without having to go back to airport. Whole process from arrival to departure actually only took 45 min and included filling out the forms, photocopies and photos downstairs. I was quite surprised. Maybe I was lucky this time? Officer was very friendly, understanding and helpful as I had made a couple errors on the form.