Buying links on BHW
I've wasted too much money on links here. Like a lot of other members, it took a year or two before I realized that it's basically just overpriced Fiverr links. I've tested both cheap and expensive services, old and new. The wake-up call was when I bought one link for 150 dollars, and then found the exact same links on Konkerr for 30 dollar.
Ditching one of my first sites
I published some shitty articles that I had written - but it still made money! I even received quality backlinks. I realize now that the site had very good potential, but I didn't see it back then, so I deleted it and let the domain expire. Might pick it up now and give it try again.
Not spreading enough
I had two sites, both earned good amount of money. Both in health niche and both got hit during the medic update. It was a valuable lesson, but for a couple of months I was in panic mode. Now I have spread the risk by having several sites bringing in bread. It made me understand that things can change overnight and relying only on Google rankings is a dangerous game. Several income streams is the thing.
Spent too much time on the wrong keywords
Also a good lesson for me. But I spent too much time and energy writing articles that were not for buying keywords. Too many evenings, nights, holidays and weekends was spent on writing articles that didn't generate half as much money as it would have done if I aimed for buying keywords. It beefed up some of the domains though and I could, thanks to that, bring in big money from sponsored posts.