Your single most significant purchase mistake?

(never have your listening chair in the center of the room if your speakers are against a wall).

Completely disagree.  Depends on your speakers and your room treatment, or lack thereof.

A Cambridge Audio EVO 150. I liked the way it sounded and I liked the understated appearance and compact form factor, but the streaming software (StreamMagic)  interface was irredeemably awful. I couldn't live with it and I was able to  persuade the dealer to take it back and I went back to using NAD/Bluesound.

Biggest mistake was dealing with Mike Powell  

FWIW, I really like Mike's foil power cords.

My worst mistake was commissioning a pair of custom speakers from Mike's audio-show buddy Ryan Scott of Vapor Audio. I never got the speakers nor a refund, what a nightmare experience.

A MAC c26 preamp back in 1976 . Couldn't wait ti get rid of it

Dynaco MK VI tube monoblocks, 120 watt each.
Unless you are up to doing a serious mod of them, stay away.