My first one was going to graduate school. I underestimated my loans a lot. Learned to talk to people before making giant financial steps. I also used throwaways to ask questions in this subreddit which made me fall in love with this place.
Second one was buying expensive WiFi because of not listening to this subreddit's advice. After paying too much for internet for my business I finally took most of the user's suggestions here and decided to buy a hotspot device. This allowed me to replace my office's WiFi & I also replaced my WiFi at home too so I owe everyone in here a BIG thank you for that one.
So please comment below and let me know what your personal mistakes were, thanks!