Joining a startup as employee no 1. It was my worst investment. I'll be happy to merely get the value of my work hours back some day, even in the best case that the exit will be quite a successful one. I don't expect to get more than what I could have made as a freelancer during those years. I'm now making way more money just working. Simply buying the shares would have been cheaper for me.

Now whenever I do the math for accepting shares over money, it's usually not worth it.