Your single most significant purchase mistake?

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.

@Jasonbourne52

That's a great find for $20! Mine is the TFM-15CB too.

On eBay a few years back I bought a TFM-35 NOS in the box for $450. No pictures, just the outside of the box. This I had compared head to head with Don Sach's Kootenay. And, yes, the Kootenay was the better with more body...but the Carver had a slightly prettier top end. The Kootenay didn't murder the Carver...in fact, in a bizarre sense, they sounded somewhat similar. They were both running through a Don Sachs preamp. Also the Kootenay had only 50+ hours on it and would surely sound better in time.