Your single most significant purchase mistake?

QSA Violet fuse - 711.00 down the drain. Bright, analytical, and non-musical. Gave up after 2 months and re-installed the stock fuse for 2 weeks while waiting for the SR Purple. The stock fuse beats the QSA garbage hands down and the SR purple version is everything Oregon Papa (RIP) said it was. A game changer at close to one-fourth the cost.

@ted_denney , simple. Mark Levinson the person sold the company. I also think John Curl has had little to do with the company for decades. There are maybe a handful of audio electronics engineers as good. 

As a matter of fact, there are many pieces I regret buying even though they were learning experiences. Top of the list would be the Transcriptors Vestigial Tonearm. I would be polite calling it garbage. Here is a twist. What about equipment that everyone else thought was garbage but turned out in your experience not to be? I put the Phaselinear 700 in that category. It was not a bad amp relative to other SS amps of the day. It made Crown amps sound like ----. If you needed the power there was nowhere else to go. I tortured mine for 10 years and it never complained.

Bob Carver had his hits and misses. His ultimate line source speakers were big misses and so has been every other dynamic speaker designed that way. The Nearfield Pipe Dreams are another example. If you want line sources you have to go Planar magnetic, Ribbon or ESL. 

NHT 3.3's. I bought them after Corey Greenberg's rave review. I could never get them to sound good in my house. I moved on to Vandersteen 3's.

Mid to late 80’ s, my Harmon Kardon 730 blew a channel. It was 10 years old, so I bought a Luxman R115 receiver. Specs were great. I even heard it in the store. However,when I got it home a set it up, my first listen was a giant letdown. The bass line consisted of the bass drum only. Speakers were original large Advent. I sold it within the year and got the HK 730 fixed and enjoyed it for another ten years. I still cringe when someone recommends  Luxman.

This was a great lesson. It was the only piece of gear that I made such a mistake. And I had listened to it. Since then, I have put together what I believe is a reference quality system without hearing anything except the speakers.

My mistake was in not buying something- two 10B tuners- $1,200; that was for the pair-