I am a sucker when it comes to spending money. I only spend on something I truly believe I need it. And I buy stuff only if they exceed VFM and that too on deals. Recent examples include buying Moto X only after 2nd gen came down, and the price dropped to 66% of original price.
I wouldn't say spending money on poster of Back to the Future was a dumbest thing I paid for, but definitely the closest I came to spending money which probably wasn't really necessary.
Though in an alternate universe, I would have spend a lot on old arcade game, probably an old arcade video game machine right in my room, and a collection of superheroes toys and every science fiction books that I can ever find.
Not sure if it's dumb or not, but I spend quite a lot of money on books; in fact, entire series of books which I have already read, just to stock them up in my bookshelf at home. I like the feeling of having a library of sorts to myself, even if it means I might not read the same books again. Weird, eh?