Your single most significant purchase mistake?

They also got ALL 200 of my vinyls collection… I am now 68… just can’t find either now

Paying a professional to create a digital room correction filter for me.  I figured out later that I needed to swap the position of my couch and my speakers (never have your listening chair in the center of the room if your speakers are against a wall).   It seems my system is too revealing - any digital attenuation comes at the cost of a palpable image, lower dynamics, less rhythm & pace.

(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.