That earlier thread on favorite products is a roaring success, here's the thread for your worst experiences with a product, food or service — it simply didn't live up to the expectations as advertised in terms of performance, taste or competence.
The list may be just as long here. Obviously each person's experience varies with even the same product or service may differ.
Off of the top of my head…
*KaBoom Bowl Blaster Kit. What was I thinking? This is a foaming cleaner that's supposed to bubble up to lather the sides and rim of the bowl and make scrubbing minimal or unnecessary. So Kate and I put the blaster in, and instead of watching it foam up and fill the bowl, it fizzed up about an inch and then just stopped. The bubbles didn't even "scrub" for that matter.
* Neuton Cordless Hedge Trimmer. Boy, this was feeble. Kate and I are big fans of the Neuton Electric Cordless Mower, so we figured this addition to the lawn tools would be helpful. That didn't work out. It really couldn't cut shrubs unless they had really thin branches and the power drained so fast from the battery from the tool laboring so hard to cut that I went back to the manual hedge trimmer. Eventually I just got a corded one.
I'm sure you'll have more. Have at it.