Believing that someone will agree to become your co-founder.
When I was planning and already making the first steps of building 356labs, I knew that I need a co-founder. A designer. A senior, senior designer. I told myself:
That’s easy! I know the right person. We are friends and there’s no way she won’t join me!
I called her and we talked. Turned out she not only didn’t want to join.She was not even interested in graphic design anymore because she wanted and already started working in a tattoo shop and now that was “her life”!
Hm…! That’s kinda not what I expected and what I was completely sure in. Now I needed to think and search for another “will do it for sure” person and in the meantime I was losing time. I needed someone to design various things for the web site (yes, I started the company based on the Lean startup methodology, but still — there were things that needed to be designed) and I needed it as quickly as possible.
So, my lesson for you here is really simple — in case you decide to start a company with someone — make sure that this person/people actually want to do that too. You may think they will want to join, but is that really the case? Communicate early and find out.