Cell phone antenna boosters
Many years ago I actually bought a device advertised on TV to improve TV reception by somehow "connecting" your antenna to your house wiring. It came with a transformer-looking plug and wire to attach to the TV antenna. Of course it had no impact whatsoever on reception. Frustrated, I cracked the plastic case open just to see what magic lie inside. Guess what I found. Nothing. It was an absolutely empty plastic shell. I consider that $29.99 tuition paid in the University of Life. No one ever went broke overestimating the gullibility of the buying public.