What’s a mistake you made by being impulsive?

Never make mistakes.

Overconfidence: the perfect example of the über mistake, right there. 

So, a couple mistakes as I built my system up from nothing. 
1. Bought speakers which were too hard to drive and then had to figure out how to get a powerful enough amp. Way more than I wanted to spend on amps, so I needed a more sensitive speaker; luckily, the maker was willing to upgrade me before the end of the build cycle. 
2. Didn't start measuring room with REW and microphone unit later and then dealt with room issues, last. The smart thing would have been to buy the $100 microphone and measure the room, and then consider equipment which might be able to work in that room. For example, my room probably would benefit from multiple subs, but I could only know that after I already purchased a single good sub.

I am impulsive by nature, but the high cost of and desire to alway purchase far above what I can afford has pushed me to research the living daylights out of every purchase... so can’t say I have had a significant bad one.

Nothing egregious, but I've bought sets of NOS tubes after a few drinks.  I've since learned to stay away from any good tube site after imbibing.  I really don't need anymore 12au7s or ax7s!

I bought a high end surround sound preamp, big mistake. Mixing analog and digital on one chassis, simply a bad idea. OP you need a subwoofer and isolation platform, forget the panels.

I would say impulsively buying and flipping cables for a few years, I eventually made myself stop and very happy with the loom I have now.