Here's mine:
When I upgraded from a 10 gallon to a 29 gallon aquarium, I unknowingly created the perfect recipe for greenwater. I'd just read an article on the benefits of anaerobic bacteria (which hadn't mentioned any of the dangers of these bacteria). So, I was delighted to find a sand substrate labeled "inert and safe for freshwater aquariums." Of course, it turned out not to be inert, releasing uncontrolled levels of minerals into the water. I also got help from a fish store employee, who asked if I planned to keep plants in my new tank. I said yes, I had a baby tears plant that was doing well in a 5 gallon tank, that I wanted to transfer. He told me that if I had any hope of keeping any plants alive, I needed "at least 2 Watts/gallon" for a "low-light" aquarium. He also sold me a micro-nutrient fertilizer, which he advised was "all I'd ever need."
So, I created an intense-light, sparsely planted tank with tons of minerals, low CO2, and almost no iron. I kept the light on for 14 hours a day, thinking that this was necessary to keep my single plant alive. Instead, I had the worst greenwater outbreak you can imagine. Unsure of what was causing it, I tried shutting out all light to the tank, killing my plant but not affecting the algae, at all. At this point, I couldn't even see my fish, and I was getting desperate. So, I did such frequent water changes that I uncycled the tank and killed some fish (including my favorite - a 2 yr old horsefaced loach). I was finally persuaded to try an algaecide chemical that contained copper and killed off my bamboo shrimp. After all that, and still no improvement, I removed all the substrate, filtered out the sand, boiled it, and put it back in the aquarium. The green water cleared up immediately, and some of my fish actually survived.