Your single most significant purchase mistake?

I purchased a used, first generation McIntosh MC275, one which had a "hard life" before I purchased it.  As a poor college student, I could not afford the repairs it needed and ended up taking a bit of a loss selling it "as is".  To afford the MC275  purchase I sold my pristine MC225 amp.

It was my senior year of high school. Money was hard earned. Some Bozo showed up with a load of JohnZer speakers for sale. For $380, I bought a pair. I cannot begin to describe how bad they really sounded. To me, they looked pretty cool. This eventually led me to build my own set of speakers from a Speaker Lab kit. It was a clone of the Klipsch K Horn. A very complicated build with odd angles to cut for the folded bass horn. They were not great either, but a huge step up from the first pair that I got so cheated on. I already had an interest in audio and music and this experience, well, it gave me experience. Same thing happened with the first electronic kit that I bought too! I just remember how cheap the parts were, but it sounded better than you might think.

 I will say this>> I have since made many speakers and electronic kits. The enjoyment is great, and you can, with the right kit, make impressive equipment, but it is not for the faint of heart. Honestly, there have been many times that I built something myself so that I would at least have a better control over parts quality. I find it surprisingly difficult to find a product that is well designed, well built, and sounds great. 

I tend to put myself through a brutal, intense, torturous, and lengthy process of questioning my own perceptions of demos before I splurge on significant new gear. It’s terrible and unenjoyable, and it usually ends with me questioning the nature of reality.  But, for my pains I’ve never bought anything significant that I regretted or let me down.  Other than a recent Innuos PhoenixUSB reclocker that I should have bought new instead of used.  Minor mistake in the end.