What is something really stupid you purchased that turned out far better than expected?
I bought a piece of 1.5 inch stiff foam to try to fix a sag in a bed. It didn't work but having that thick piece of solid foam around has been a life saver.
Need something flat to put a laptop on? Throw it on the foam. Going to be doing something that requires you to be on your knees for a while? Get the foam!
It went from stupid purchase to something I'd gladly replace if it broke.
One time I was in a hardware store and I picked up a pack of those ratchet straps that you use to hold down loads for like $25. I figured that I had a pickup and I'd probably need them "someday". It was an impulse purchase and I felt really badly about it for a while.
10 years later and those straps are worth literal gold. I have gotten so much use out of them over the years it's kind of mind blowing.