Not too many for me, mostly because I grew up poor as heck and I do a lot of research and buy quality.
Still, everyone has a thing or two....
I bought a starter for my 91 Bronco from the shady local Ford dealer when it went out the week I was supposed to move to my new home. 6 months later it failed. Got one from the parts store near my new home...huh, lookie here, this doesn't look the same.... well the one from the shady dealer was basically a chinese egg beater motor they put in a Ford box. I called and cussed a bit but they got their karma when the dealer shut down a little while later when their reputation caught up to them.
I bought a floor jack from Walmart(hey I was young gimme a break, this was in the 1990's) and the linkage for the pump handle failed after three uses. I welded it and used it more. Thinking back, I should have known better- if the linkage was so cheap, what about the rest of the thing lifting my truck off the ground?
But the worst was a trailer. A BIG utility trailer. It was a home built job, but the guy had bought legit trailer axles(1500lb each, in tandem!) with the "wide" "fat" style tires. I used the thing quite a bit, it had 3/4" marine plywood floor with metal brackets he made for the edges so it was super flat and nice. Trailer was a "4 snowmobile" size, worked great for landscaping things and moving brush, lawnmower....
Then about two years in, one of the floor panels came up a bit. Figured a bolt broke. NOPE, removed floor. Found out the guy had used pallet racking cross members for the structure under the floor. That type of thing is NOT for outside, not really thick wall, and hollow. Well, the tops had rotted where they contacted the floor and there was no way to reattach the floor now. I could have cut and redone the whole thing, but I cut the axles off and kept the plywood and gave the axles to my wife's Uncle for a a few favors he had done for us. Not the end of the world, I got some use from it, but it cost a lot more then the use I got! Last home built trailer I ever used much less bought.