Your single most significant purchase mistake?

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. 

SAE integrated unit. It was their solid state all in on. Looked cool and did sound good but. The first one I received all the inputs were wrong volume said balance cd said tuner snd do on. They actually made me pay the shipping to send it back. After six months the left channel went out. Dropped of at local repair and he told me he could fix. He did fix but also regretted taking on the job. Hevhad to  Re-solder the entire board. Said it was all cold Soldered and very poir. Worked for aboutvten years and one day I hit power onlybto hear a click and never powered up. Unpluged everything and placed in garbage!