Worse performing/biggest loss on a stock, by percentage, that you have ever had?

What is the worst performing company that you have had in your portfolio? Do you still believe in the company, do you still hold it or did you sell for a loss? In hindsight, where do you think things went wrong? Knowing what you know now, what did you learn from the loss? I have a few: * ZG (Zillow): -57% I still hold this position. I think real estate is due for a disruption in the insane 3% closing costs and dont think real-estate agency offer a service worth the cost. However, in hindsight, I should have sold based on their failed entry into actually buying/selling homes directly. I liked them because they were a low asset tech company in a market prone for disruption. They converted into a high asset high risk area and started to compete in a legacy industry. Basically everything that I liked about them, they actively managed to pull in the opposite direction. I still hold because they have admitted their mistake and are returning to a tech and marketing company, and I like the new CEO. But this remains a low conviction. I will probably use it at the end of the year for tax write-offs * KLR (Kaleyra) I think this was negative -60% when they were bought out. Dumbest investment I ever made. I simply didnt understand the business well enough, and I averaged down a ton. I dont even know what lesson I learned here... other than only to invest in things you really understand. And that the best company to invest in, is usually already a winner you current hold in your portfolio. Dont be afraid to average up and reinvest on your winners. Take caution when averaging down, its possible the market sees something you don't and is more rational than you like to believe * CRSR (Corsair) -55% this one hurts because it wasnt a small position. I didn't understand the balance sheet and overestimated their branding and pricing power. I also was comparing their evaluations to "peers" like Logitech which were actually were quite dissimilar. Hindsight is 20/20. What horror stories do you have in your investing history? What did you learn? Maybe we can learn from them too >"A smart man learns from their mistakes, a wise man learns from the mistakes of others" \-some guy