Your single most significant purchase mistake?

Anything with the name Arcam on it....

Rel s812 subs. Without a doubt the worst purchase I ever made. Kept believing all that hype but when it came time to perform, although they were fast, they just didn’t go low enough and were incredibly difficult to integrate. (I guess most likely because there’s no fine-tuning on the back of them)

Seeing how they were overpriced to begin with, I lost a lot of money in that transaction. 

Regardless, lesson learned, will never buy on media hype ever again.

@zavato  - Mr. OP, did you provide an answer to your own question? Curious, didn't see one. Were you thinking about a mistake you had made when you posted this question?

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.