What’s a mistake you made by being impulsive?

Add Monster To the marketing hype companies like Bose has been. (Although Bose finally came up with a nice sounding headphones... although not remotely accurate).

McIntosh MA252.sold it after 90 days. Very underwhelming. Made some cash though.

In my case, you should have asked, "which impulsive purchase did you regret the most" as soon as you bought it and I could answer that question easier.

In mid April I purchased a Rega Planar 3 w/Ania MC cart to replace my Heybrook TT2/LVX/OC-9ML combo.  
While I was breaking in the Rega I serviced the Heybrook myself in order to sell it.  
After setting everything back to spec I had a listen and guess what?
It sounds considerably better  than the Rega combo which has 30 hours on it.  Guess I should have done that first before I bought the Rega.
The OC-9ML has around 200 hours on it... it's a keeper.
The Planar 3/Ania is for sale.

Very nice thread topic, well stated! You should get a lot of varied feedback. Here's my experience... 

I'm sure there were at least a few dozen times (even though I try to discriminate in regards to what I will/won't view) that I saw movies which were unremarkable. Also, I have eaten many meals during my lifetime that were not culinary delights. I hasten to add that my wife is a terrific cook, and there were only two times, as we were about to leave for vacation, and she scrounged in the fridge to make an unusual meal simply to use up perishables, commenting, "This will not taste good." She was right, but those were the only two times what she made was not delicious. 

I have been through dozens upon dozens of pieces of gear, and in some combinations they weren't impressive. In others, glorious. However, everything I did in the lower budget realm (i.e. $5-10K) pales in comparison with better rigs. I could consign all of the sub $10K rigs to the unimpressive category. 

Is it impulsive to buy to upgrade? Sure, why not. We're all doing that. None of this is needed. Pretty much all of audio is a degree of impulse buying. 

The first few times you don't get a good match it is deflating. After a while, if you keep up with system making, you realize not every combo is golden. No biggie. It's when you think you have to get every dollar maximized, as though you can't make a mismatch, that you are going to suffer. So, suffer. A realistic attitude will realize the plethora of combos WILL yield wildly different sound, and the spectrum of performance is WAY beyond what you think. 

Finally, it takes a lot of work to get some products to sound good, but some never will. Those you can usually screen out due to inherent design/build characteristics, if you know what to look for, and your preferences. Ignorance kills. But, how to get experience? Make mistakes.  LOL I would not take back any of the experiences I have had, for they shaped my development as a System Builder. I learned so much from all the different forms of systems. 

The worst? Determine that you have to find the perfect piece for your system. Think it must sound far better with little effort. Great way to set yourself up for problems. :)

willgolf, good point on impulsivity on selling in order to buy. That is for sure a killer! BTW, read my review of the Legacy i.V4 Ultra Amps at Dagogo.com; class D is VERY different now than several years ago. It is glorious with horns - and everything else!