I am assuming the spirit of this question is what is the worst speaker you've heard, that should have been good. So, I'm not including computer accessories, built in speakers in clock-radios, etc.
So, the worst for me was a pair of B&W towers (exact model I don't recall; picture of something close pasted below)....they had the yellow kevlar midrange without any visible surround. Now, while the demo occurred at Magnolia and so I'm sure conditions were imperfect, the salesman played Adele. And because my wife loves Adele and I heard that same track 100s of times on my own system, I had a good sense for that recording. There was an unmistakable ringing in her voice--like a resonance or vibrato that was not supposed to be there--that drove me insane. It wasn't the room because other speakers did not show it. To this day, I cannot look at a yellow-coned B&W without cringing.