It was a mistake of omission.
There is this startup with a promising but unproven technology. I spent weeks searching the business and the market. I got to point of pulling the trigger. It was at a price that I was comfortable with to put in a small stake based on my calculated risk/reward, knowing I could lose it all. I literally had my finger on the "buy" button when I hesitated, wanting an even cheaper price. The next week, price started running away from me and has never returned. If I'd bought it, it would've been a 3-bagger in less than 3 months and would've taken care of my 2013 return single-handed.
It's been really painful watching the price running further and further away from me on a daily basis. Trust me. It's painful. Even having reflected on the lessons learned, knowing probability plays a role here and drilling on it does me no good, I can't help agonising about it.
Emotion can kill an investor.
Another lesson: net-net mentality is sometimes wrong.