Your single most significant purchase mistake?

Emotiva Amp/preamp.  Distortion distortion.  That what I get for succumbing  to sales pressure and not researching.  
 

@retiredfarmer, I felt my Studio 100s were lacking, but then coupled them with Cambridge CXNv2 music streamer and Parasound A21.  They never sounded so good.  That said I really want a Sonus faber Olympica Nova for there tweeters.  Fell in love after hearing them 

OK, this is interesting. There are 2 products listed here that I've enjoyed immensely.

I previously owned the Reference 3A MM DeCapo(i) for more than 5 years and enjoyed every minute with them. I can say that I had a pair of highly updated Reference 3A Veena floorstanders that I didn't love. They may have been great for someone who prized detail over musicality, but they  didn't work for me, in my system, with my electronics.

I've also owned 5 Rega products over the years, including my current electronics - Rega Elicit-R Integrated amp and Rega Saturn-R CD/Transport/DAC. The Rega combo has essentially stopped me looking for different electronics. I think they are a great value and they are both very musical pieces IMHO. 

I'll be interested to see what other 'mistakes" people have made, that I love...

Probably a Carver pre-amp back in the 90s. Someone here wisely advised me to lose it and I did and it was a new day dawning.

Carver amp was fine applied properly.  Still have Carver tuner.  Still very nice!

Back when I was young and dumb(er): A Garrard Zero 100. Junk.

OK, this is interesting. There are 2 products listed here that I've enjoyed immensely.

Someone mentioned a pair of speakers that I just recently acquired as a "mistake".  I've been really enjoying them but found them a bit "bright" when pushed. 

Over and over the comment I've heard made most consistently about them was they "are very revealing of the source" and the source I listen to most is vinyl.  I've been running my phono stage with no loading since I bought it and love the detail and "air" and pretty much everything about it.  I decided to try adding some loading last night, and everything changed for the better.  Did these "bad" speakers suddenly become "good", or was it operator error at worst or lack of properly tuning the entire system at best?

I think I need to still play around with loading settings, but my point is that some gear is more system/room dependent than others.  I have a feeling some of these "mistakes" were more mis-matched components than something inherently "wrong" with the gear itself. 

In most instances I buy things without having ever listened to them and haven't made too many "mistakes", but there have been a couple along the line.  Even those were more listening preferences than there being something inherently "wrong" or "bad" about whatever it was.  One person's "bright and edgy" is another person's "highly resolving and detailed".  Another person's "warm and engaging" is someone else's "boring and subdued".