Your single most significant purchase mistake?

Sonus Faber Sonetto III speakers - not detailed and transparent enough!!

NAD C275. Not an expensive purchase but was my first real foray into separates. Huge expectations. I had just purchased a NAD preamp and was looking forward to audio bliss.

Then for kicks, I put my old 1994 Carver TFM-15 in there and it killed the NAD. And I mean it totally murdered it. I had always used the Carver with CD directly into it. It sounded OK. But with preamp in front of it, WOW.

Break-in did not help...sold the amp the same month. 

It scared me into really vetting gear before I buy. 10 years later, I haven't had a purchase regret since.

A turntable.

SOTM Dac and server.