Ever bought gear that you regreted either right away or after you used it on a pitch?
I've got tons of clothing that I don't think I've ever worn after purchase, a NF Denali fleece springs to mind.
Actual gear though, I think mine would be a Mammut Flash 10.5 rope. Not because it's a bad rope, but it weights a ton and I bought it to replace one of my aging ropes but it's just too big. Ended up buying another 9.5 so I have the 10.5 sitting around. Note to self, just because something is on sale doesn't mean you should buy it.