We've all made costly mistakes im sure, in fact I believe thats what makes me a frugal person. A couple examples for me...

Buying a new car. Thought the 0% interest sounded like a great idea until all the extra fees were added. Freight, handling, emmissons tax, administration, credit app fee, optional warranty and accessories, etc.. Final price seemed nearly double the sticker price in the end. Two years after owning the car before loan balance below sticker price. Also my next vehicle will hopefully have the same tire/rim size so I can keep using my spare winter tire/rims rather than buying new ones.

Another somewhat costly/wasteful/inconvienence mistake. After our son grew out of his toddler bed we decided to go straight to a double. Of course at the time we didnt realize he was still going to be wetting the bed 4 nights a week at 6 years old. Should have put him in a single bed, would have cut the laundry load in half. Now we have 3 bed sizes, single/double/queen, lots of various linen sizes to over complicate matters. Currently addressing the issue with doctors/pediatrician soon to be resolved hopefully.