Hmmm Let me see. Top of mind are Vibrapods. Some change in sound, nothing groundbreaking but the worst part is they somehow leaked this weird liquid that stained furniture. Gross.
When experimenting with shelf materials many years ago I tried wood, acrylic, and granite. The granite wasn't quite thick enough so it rang and rang more with the combination of spikes. Sound was tipped up and harsh. Then my first bout with acrylic was not as bad but when I tried another type of acrylic, that was much better. MDF was flabby, hardwood okay, butcher block much better than solid hardwood. Went with differing layers but the guess work burned me out. I ended up with ready made passive shelves CLD with acrylic, sorbothane and bitumen coatings on the acrylics. Never looked back.
Volt reader. You know, the ones that look like 80's alarm clocks with the bright red numbers. Obviously does nothing for the sound but it did serve as a cause of audiophilia nervosa. Couldn't take the distraction so I chucked that.
"Power Conditioners" My first ones were nothing but glorified power strips with some caps in them. Made the sound gray and dull. No they weren't Richard Grays but come to think of it that would've been really funny in an ironic way if they were. Had another one for a second system that was made in China. Rolled off the top end and rolled off the bass. Maybe the designer thought making everything sound like Bose 802s was a good idea. It now resides with my Pioneer DJ rig which funnily enough benefits from its weird character.
Blu Tack. Talk about a crap shoot. Too much? Too little? Where to place them? Didn't have many options way back then especially with little kids running around. Saw the errors of my ways when I finally tried damping pads actually meant for monitors.
Lead shot. In my opinion still the best thing for mass loading stands, bird shot to be precise. So what went wrong? I had to send some to another province for a client and almost got arrested when the security dog sat on the box making the K-9 cop think I was shipping ordinance. I think that qualifies as making things BAD.
Steel Ball Bearings. Give me Tungsten Carbide or give me death.