Biggest Regret as a Collector

So this is based on a contest that MVC ran on his Youtube page after reaching 100 subscribers. At the time, we were recent friends, and he encouraged me to make a video response, but I didn't really feel I had anything interesting to say on the theme, since I had been back into the hobby for under 2 years after being away probably close to 3 decades. That said, I did have a recent incident that definitely qualifies as an appropriate response, even if not for the contest's sake, just for the sake of sharing our collecting mistakes. I'm hoping others will chime in here with their own collecting mistakes, but to get the ball rolling, and finally answer MVC's original question: Not too long ago while at Walmart I saw a bunch of 2018 Topps Update on clearance. This was before the Soto and Acuna cards really peaked... it was still the regular season, sometime during the summer. Not really looking for anything specific, but only because a fat pack was like $3.29 on clearance, I bought a few. I pulled a Ronald Acuna Rainbow Foil. I knew it had some value to it, but didn't think it was all that big of a deal. Couple of weeks later, while selling at the flea market (I don't sell baseball cards specifically, usually just random collectibles and odds and ends I pick up for cheap at garage sales and can flip for profit), and being in need of some extra cash for various things that came up (car issues/relatives birthday etc.) I decided to sell it. When I checked Ebay sold prices at the time it ranged from 25-30 dollars. And I sold it pretty easily at the 25 dollar mark. I was happy, I still had the regular update rookie and some chrome rookies (and thankfully still have them). However, about a month or so ago there was a sudden boom in the Acuna cards..... you all know the bat down card everyone wants, etc.... and even the update packs and blasters had more than doubled in price as everyone was looking to grab those Acuna and Soto rookie cards. Well, despite knowing better, I decided to look up the selling prices for the Acuna Rainbow Foil that I had previously sold at market value for $25. The sold listings now were at $150 and sometimes nearing $300. And that, so far, is my biggest collecting mistake since getting back into the hobby. Now truth be told, if I still had it, and could get a couple or few hundred for it, I'd probably sell it right now and re-invest the money into an Alonso psa 10 auto for my PC. Anyway, that's my biggest regret. What's yours? (For reference, here is the link to the original MVC video I mentioned in this post): https://youtu.be/9UtkmchGz4w