Your single most significant purchase mistake?

Not measuring twice and buying an extra 1.0M worth of interconnects and power cords. It was compounded by the fact that I have mono blocks and two subwoofer amps and they are sited 12 feet from the preamp. That's a lot of extra cables/cords--and they are very fine (and expensive). Ouch. Wish I hadn't done that.

 My ex wife

Martin-Logan Monolith IIIx. Ultimately, my bad. Magnificent speakers that could never integrate into my room. Broke my heart. Sent them to Japan to an eager buyer, who paid the shipping! I got off easy, money-wise.

Raven Audio Blackhawk. This product was over-hyped on these forums. I bought one and found it to be below average. Sold it at a significant loss. 
 

 Peachtree 125 - bright, bright, bright  

bowers and Wilkins CM10s - sterile, thin midrange. Sold it within a year. 

Forgot to add Topping D90 dac. This was supposed to be best measuring dac that was a giant killer. Hyped up on ASR. Boring, sterile, etched sound. No soul.