Your single most significant purchase mistake?

i noticed the recently initiated thread on this mike powell fellow and his youtube posts were abruptly taken down... i had posted details of my unacceptable experience on that thread dealing with him

Biggest mistake, probably buying a fabulous Threshold T-200 power amp used and finding out it didn't mate with ML SL3 electrostats. It was twenty minutes of music and you stopped...had to sell it...still have the SL3s (in home theater now).

As a surprise, the Carver AV-705 multi-channel amp sounded much better than either my brother or I expected and he used it a while on Thiel 2.3s (late 90s). It was a steal, bought used.

Jeff Rowland Model 2.  Listened to it in the store.  Sounded much better than the Mac amp I currently had.  Brought it home, and it sounded much worse.  That was when I learned you needed to match the equipment to the room.  I still remember my wife telling me to get the old amp back in the system.

B&K 5.1 Surround Receiver

Sounded good but when it had a problem there was nobody to be found. I left it sitting on  the service counter at Listen Up in Denver and walked out the door.

Thus far it would be a miniDSP unit. I told them what I had, what I was trying to do and they recommended a product which after purchased was told by them it wouldn't work in my system.