I would not call financing anything a "mistake" as long as the borrower knows what they are getting into and is comfortable / prepared for it.. I have bought stuff on financing many times where I could have paid cash and not paid any extra in interest / loan fees, but those costs where low enough to be a good insurance to me against having no cash on hand for the things you can't finance or don't come with nearly as good of financing options.
I have our TT on a 15 year loan just for that off chance that stuff goes so badly one month I only want to pay a tiny payment. The bank has pre-printed blanks on the payment form for additional principle, so I don't think paying extra is any sort of a bad idea and I am comfortable that the extra will have it paid off in 6 years or less.
