For me it's those little fibre pots for starting seeds in. They dry out so quickly that you can't provide a stable place for good germination but if they are moist they fall apart in your hands!!!
They seem like such a good idea but they're really not?

I once bought some aeration sandals. They were no more than soles with spikes on that fastened with a strap over the top fo your foot. Absolutely useless.

Aren't all sandals/flip flops good for aeration? I'd love to see a photo of how they were marketed as useful...
Or was it downward spikes for lawn aeration?! Which is even more hilarious!! Lol!

Weed barrier, lol! ain't do diddly!

Those little tips you put on a water bottle to keep a pot moist. They either don't water the pot or all the water empties quickly.

I had some of those fibre pots & got rid of them for the same reasons as you Darren8

Bulb planting tools. The one we bought (expensive variety too) bent the first time I tried to plant bulbs under our lawn with it.

Ditto re the little fibre pots and the bulb planter, although this does work in soft ready prepared soil where you really don't need it!

For a bulb planting tool, I got a piece of a tree limb, put a point on it. It works just fine and cost nothing.

Eight thousand year old digging stick! Sometimes the old ways are best! ;)
For me, it is those covered propagating trays: The seeds sprout well enough, but then you spend weeks hardening them off so they can live in the open air, much less the garden. They're only good for starting esoteric plants with dust-fine seeds, such as African Violet, or sensitive softwood cuttings--not something the average gardener messes with.

For me it was one of those forks with a short T shaped handle. You were supposed to push the fork into the soil then twist it from side to side to loosen the soil. All it did was give me a bad pain in the back!

I also agree about those fibre pots. Useless.

I think mine was a gadget for aerating the compost bin. You pushed it into the middle of the compost and then when you pulled it out again two little wings were supposed to open and lift the compost . I thought it was a brilliant idea but it didn't work for me.

Membrane supposed to keep down weeds. I would never buy it again because perennial weeds spread far more quickly under it. Now I use newspaper before putting down mulch where I have plants growing and I have recycled carpet on the paths where I spread fresh wood chip.

Anyone interested in a cheap bulb planting gadget?
Never used...
It reminds me of the anecdote about the guy that ordered a limited edition Harry Potter wand & got an expensive stick by return!