Your single most significant purchase mistake?

Everything I ever bought from Rega. Never again. 

Emotiva pre-pro

Sold within a short time.  Lots of issues.  Sounded OK when it wasn't constantly switching "modes".

@stereo5 , it was a Clarion years unit (production run of 1992-1998).
I'll admit that Mc has built some "duds" over the years in an attempt to chase trends, but feel they hit the mark more than they miss it. My C2300 is certainly a keeper.

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...