Your single most significant purchase mistake?

Martin Logan Sequels. 

Smallest sweet spot ever.  Move your head 2 inches and it sounded like you put a bag over it.  Thin, brittle sound.  MLs may have some good speakers but those were not it. 

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!!