What’s a mistake you made by being impulsive?

A good question for sure. I lusted after a particular brand X amp made by a boutique company until I had the money to purchase one. I also purchased the preamp....then another preamp. I won't disclose the name here but the experience with the quality and performance was poor and the guy who makes the equipment was an arrogant jerk. I ended up losing lots of cash (mind you this is a poor's man's system bought with hard earning money) and I will be much, much more careful in the future. My current system has had no problems, while I see other people buying brand X and still having QC issues. The term still applies "buyer beware" to audio equipment.
BTW I did not immediately regret my decision.... but rather learned that some hyped up brands do not always meet expectations. In the case of poor workmanship, you must determine how much a company will cut corners to sell products.
A good example of this is by Danny Richie at GR research. https://youtu.be/1S-jTJK43t0

Never make mistakes.

No serious regrets really. But listening to the hype begat too many lateral moves.If I only knew then what I know now about component matching,build quality, and acoustics. Never having large amounts of money to throw at this hobby probably saved me.

Like jtcf I have no regrets, but I wish I could go back to my vintage Adcom and Parasound amp days and replace them with something more sublime. Had a pair of Bozak speakers back in my college days and never figured out until they were gone that perhaps more power was required. So I never knew if they just really sucked or would have opened up with a much beefier amp. Never sold a component that I regretted selling which is nice.

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.