1. Back up the back up. I thought I had everything under control. As soon as I could I make a copy of the SD card on my hard drive. But hey, the hard drive broke and the SD card got writen over. There is never enough back up for your images. The cloud seems the safest space right now. Hard drive recovery can be a very expensive joke. I paid a 1000.- euro and did not get the images back. Seriously, back it all up, now!
2. Falling into the "Exposure" trap. People think that photography is just clicking a button. It is for them but then, why don't they do it themselves? If they want YOU to take their pictures they have to pay for your time, equipment, travel costs, etc. Free work is OK only when YOU want to add this images to your portfolio. If this does not benefit anyone but another company you should not do it. You are running a business, otherwise take it as hobby and find a side job. Besides, you would hurt the industry, since more and more people are getting used to getting images for free. Students take images for free, and not even.
3. Not having a website.
Whenever I met a photographer without a website I feel confused. Some have the argument that they are still working on it and others say they are already too busy to keep it up (yet, they have a side job and aren't following their own dream). I do believe that for the bigger clients and the long run, a portfolio is not only a symbol of professionality but a basic showcase need. Creative agencies won't have the time to meet first to look at your analogue portfolio, it is 2017.
I hope you learn from my mistakes, specially the back up one. Good luck with your photography!