Just thought of another 2 in addition to the Audiolab 8000S already mentioned. ( I obviously tried to block out from memory because I am still traumatised, 30 years after in one case ... .)
That would have been a pair of RTL TDL 3's. The biggest waste of MDF ever made? Huge, cheap and as far from a proper Transmission Line as it gets. It was sold cheap through RS. - Bad, low cost drivers in huge expanse of thin, Vinyl covered MDF. The tweeter was like a cheese grater, bass ... horrible.
The other (really) bad purchase, although not purely HIFI, was a Pioneer Kuro Plasma TV, purchased largely on the basis of WHS&V and others waxing lyrically about it at the time.
I could never get a picture I liked and the thing buzzed like hell whenever brightness increased. Speaking of which, the picture dimmed like crazy every time there was a large amount of bright screen content.
Sold it after 6 months at a considerable loss.
Far from the worst purchase ever but nevertheless something I'd rather not have bought ... a pair of B&W Formation Duo's Active Speakers, just recently.
Fantastic in some ways, bass, power and dynamics ... not so in others. Largely the usual B&W sound of bright upper mids and lower treble with boosted bass. There was a constant deviation from neutrality which imparted itself on everything and which started to grate after a while. Some of the traits were likely due to the class D amplification not helping.
Some Kef LS50 Wireless Meta's were superior in certain areas and I preferred them. However, they too had a signature which was ever present and did not conform to neutrality. Again, Class D powered to the mid/woofer ... .