Hah, this is easy. (This is a stretch of my dumb decisions from 2003-2013)

1. Decided not to work on a company with a good friend. Friend grew the company successfully, isn't involved in the day-to-day anymore, he's worth millions and is trying to think of what he wants to do next. I would have been a co-founder. All this because I didn't think we would work together well, which might have been true, but I'll never know.

2. I walked away from an interview (before interviews became 500% harder by having to know all the latest algos off the top of my head) where a guy told me he wanted to make all their Rails developers millionaires in the next year. I would have learned a lot from these guys because they were building and hyper-growing 'community' sites once every few months. The company sold to Monster.com for $400m a year later.

3. Walked away from another company (a consulting company) where I was one of the top Rails guys - I helped them build their first Rails project (they were mostly a .Net & Java shop before). Super smart and stable team, I was offered a Sr. position and profit sharing. They ended up incubating a few successful Google ventures. My good friend in the company, after a lot of years of making good connections and stockpiling away cash, went on to found a company that Google and other big names backed.

4. I walked away, after a successful contract, from a design company I had worked with on multiple occasions. I had friends there and I was well regarded as a smart developer. They 'only' offered me a Sr. position (I wanted a Dir. of Technology role). A year later, facebook bought the entire company and some of the js devs I worked with went on to create React and Flux...

I spent about a decade working on my own startups with CEO's that either (this affected all 4 of the examples above):

* Didn't understand technology at all

* Were just fuck ups that misused funds and wanted to shoestring years of product development for no reason

* Couldn't decide on how to grow a team to build a sustainable product

Of course, my startups all failed and I never learned or figured out why until lately. I'm probably in the rule of the majority of businesses failing.

So, what have I learned then?

1. Fail before you have a family or even a significant other. Failure and family are a bad mix and you will have less and less time to try to fix things, if you can.

2. Learn from others first. I'm fine with pursuing an idea, but see how others fail and succeed before you step out on your own. You'll be glad you did.

3. If someone promises you they're going to exit or you see some momentum with the team/product, spend a year with them and see what happens. Or even 6 months. Life is short, but a year goes by quickly. If you can get some win out of a year and this allows you up to be more financially 'free', do it! Once you can own your time, then the game completely changes.

4. Be true to yourself about who you really are. Be responsible for your decisions and get organized about your life. Don't float (for too long). Act with purpose and don't forget that discipline coupled with action is the most powerful thing you can be/do in regards to business (I think).

5. Pay yourself first. Just google this and you'll see why it's smart.