Your most disappointing purchase or audition?

I have said it before in other threads, but since this thread asks the question, I would have to say the Pass Labs X 250.5. I used it in single ended mode, which I was told later was not optimum, although I never read that in all of my pre-purchase research. I bought it and auditioned it with 5 different speakers in my system, and in each case, the sound was thin and electronic. It was a really shocking disappointment.

Other than reliability/service issues (decomposing surrounds on Zingali speakers and horrible Audible Illusions repair service come to mind) I don't hink that I've ever bought anything that I later really regretted (and I have bought a lot over the years). However, I've definitely heard some clunkers on demo:

I remember hearing the original Martin-Logan Sequel at IIRC Innovative Audio in Brooklyn Heights about a million years ago. That was some really bad integration of a dynamic woofer and a panel. It might have been one of the first pairs manufactured and the underdamped woofers were just a mess. I believe that later examples of that model were much better sounding (there was also a Sequel II at some point that addressed the issue). I also heard a demo of the big B&O room corrected speakers in their Beverly Hills showroom +/- ten years ago that was so bad that I'm still left wondering if one or both of the speakers was defective.

Cary SLP05. Spent big dollars (for me) to purchase it used ($4k) and I never could get it to sound up to its reputation even after rolling tubes. In fairness, the unit's performance may have been a simple case of poor system synergy in my setup.

Also, Cary 303/300, no better than a boat anchor imo. No benefit of the doubt here though.

One man's meat is another man's poison.

Magico S3. It was in a dedicated room with McIntosh gear. It was OK, but not worth the money or the hype. No passion to the sound.