I lost most money on stock trading actually. But it was back 2009-2011. I could have bought a new Lexus paid in cash with it. It has been haunting me for years since.

Then I started a cheap jewelry retail website. I did small and invested only $800 for inventory. The site ran on Magento and cheap PHP hosting like $7/mo. I got literally zero sales after 8 months. I went down San Fran Piers and saw they were selling same thing for cheap. Doh! I gave up the business and put away all inventory, which wasn't much. However, the time spent on taking photos and upload hundreds of $2 products was all wasted. It's plain stupid.