Yes I have. My 90G mixed reef went bad and crashed hard 12 years ago. I ran every test I had (and there were a looooot of tests done) and couldn't narrow down the issue. My equipment was working fine. I had done my water changes. My husbandry was good. I was, well, not happy with me. Because it HAD TO BE human error somewhere in there. Never did pin down the issue. I just gave up after that.
What did I learn? Not to give up. It's interesting too, because I'm not the giving up type. But that time I was. So 12 years later, I'm here, reading posts, watching videos on YouTube and reading articles on a daily basis. My 120G mixed reef is slated to launch this winter. I want to be armed with as much knowledge as possible before I do this again. I still feel guilty that all the livestock under my protection died out. This time around I'll know MUCH more going in than I did the last time.