Your single most significant purchase mistake?

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.

My first turntable in 1974- BIC 940 belt drive. What a POS! I think it was designed engineered ok, just poor quality of materials & construction. The fluid in cueing system leaked on early & would drop the needle on the record faster than you could do it by hand. The speed wasn't stable & the attached mat on the platter was uneven. 

I could have purchased a fully manual Thorens for the same price or an automatic Dual 1247 for about $20 more. Live & learn.