Your single most significant purchase mistake?

Schitt Aegir - Had a pair that would crash and burn while driving LSA Statement 10s and Fyne F501 speakers. Come to find out their advertised ratings weren't even close according to actual testing by Stereophile and Audio Science Review. After zero response from Schiit I sold them for a loss

Lumin U1 Mini - Absolutely horrible app GUI and way to dark of a sound for me. I sent it back and bought the Auralic Aries which it fantastic. 

A pair of Acoustic Phase speakers ($200) that I bought at Crazy Eddy's in NYC back in 1976! Large two-way boxes with a 10- inch woofer and a 1-inch mylar tweeter. Actually not bad - but not special! Sold them for half-price to a co-worker.

@sandthemall : good call on the Carver TFM15! I have the cb version in use in the front room system driving OHM Walsh Sound Cylinders. I bought the TFM15cb at Goodwill for $20!

The whole surround sound think, especially the final system, a Yamaha DVD player that was unreliable, Rotel Pre processer that never worked properly with volume that changed mid track lots of lights but no setting that was not awful, 2 channel power amp that was really quite superb and stayed as a stereo amp for 15 years until recently, a 5 channel power amp with no guts and 5 Kef speakers that apart from the centre, I still have but have no need for, were lumpy and disjointed. 

Funk LSD turntable… sounds lovely but it’s fiddly as all hell…..