Equipment malfunction might be one of the top 3, but the other two are USER ERROR and MISINFORMATION. I made many a huge mistake on my tank by not fact checking what a guy at my LFS had told me. I was new to the hobby about 4 years ago, read somewhere that I needed a pH of 8.3, so I bought a buffer after talking to the employee at the LFS... I think we know where this is going. I put the dry buffer directly into the tank, tested pH, and nothing changed. I did this until it seemed ridiculous the amount I added wasn't raising pH. Well, I was using an API pH kit and I didn't know what dKh was at that point. I went into the LFS the next day after most of my fish died, so they tested the water and found the dKh at 23... Welp, that was the last time I trusted any advice they gave and I found reef2reef. They've had most of their tanks crash multiple times, entire fish systems wiped out by disease because of a lack of any sort of quarantine system. Even observation should be minimal before putting the fish onto the store floor. Thank goodness for reef2reef. I've still made many a mistake (for usually one of those reasons), but I've learned a lot from here.