One that appears regularly in Popular Mechanics is the suggestion to drill holes in the rim of your paint can, in that little valley that runs around the opening, so the paint can drip back in. One time they published an article about all the tips they get, and they said that one just never, ever, works. They just publish it from time to time because so many people keep sending it in.
One that’s supposed to work but never has in all the times I tried really hard to do it: seeing for myself that the Earth is round by watching the sails of a boat get lower as it sails away toward the horizon. I can never see it that clearly, and I can’t tell if the faint little triangle is smaller because it’s getting lower and smaller or just because it’s getting smaller. And I can’t see exactly where the horizon is, either. Besides, I can’t see exactly how big the effect of differential diffraction is, so I’m not sure how to interpret what I see anyway. Yeah, I get that it should work, and I promise I have believed the Earth is quite spherical for easily 90% of my life. But I’m damned if I can see it.