1. Not majoring in computer science the first time I quit my physics major. If you like something, don't stay away from it for silly reasons (like thinking that school will make you un-like it).

2. Not knowing the difference between profitability and traction when I was looking at startups to work for. I spent 2 years at a relatively low-growth startup because I thought that its profits meant it was successful, when I could've learned much more at a higher-growth yet unprofitable startup.

3. Letting ideas sit too long. I had the idea for http://www.whatshallidonow.net/ about a week before I started implementing it, but was busy with job apps. In the meantime, http://nowdothis.com/ launched, and now they have all the momentum.

4. Spending too much time on super-ambitious projects and ignoring the low-hanging fruit.