Your single most significant purchase mistake?

Reference MM de Capo speakers. Everyone said they were great; reviewers and owners alike, so I bought a pair and hated them. They were soon gone.

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