To add to the list which I am sure will be developed by many far wiser than I:
The "Ownership" Myth: There is a prevailing belief that 100% is worth more than 49%. This gets in the way a lot. If by distributing 51% you are able to secure the elements needed to have a valuable business than 49% of "something big" is a lot more than 100% of "nothing happening"
The "Feature Rich" Myth: If your critical differentiation is "we will do it all integrated together" -- then it is a bad idea. Everyone -- even your "no clue competition" have plans to do it all. It is how to get there and what you roll out when that make the difference.
The "I am alone" Myth: If I read another business plan/proposal claiming there is no competition I am going to barf. On the plan. There is always competition. Think about it from the perspective of the customer's buying decision.
The "Technology First" Myth: The path to greatness is rarely having the best technology. It is far more often having the better relationship with the customer. It is a marketing game -- not a technology game.
The "Technology Only Myth": This myth is often perpetrated on this site -- the irrational and unsubstantiated myth that the world of business revolves around Silicon Valley and web-based start-ups. The vast majority of new startups are not in San Fran, or CA, or technology. Every day there are new restaurants, new medical device companies, new yoga studios, new energy alternatives . . . .
The "VCRG8" Myth: Building a company through the growth of customers is like building your body through a steady work out. Growing your company with an infusion of VC money is like taking steroids. There are always consequences and choices. Success is not getting VC funding -- success is building a sustainable company that impacts people's lives.
The "Static Market" Myth: The characteristics of your market now will be the characteristics of your market when you launch and in 3, 5, 10 years. There will be other competitors. The existing competitor will respond to you. there will be changes you can't forsee. Your plans should take that all into consideration.
The "I am the One" Myth: I know, it is helpful to think you are the next Steven Jobs, William McKnight, Mark Zuckerburg, Sergey Brin or David Packard, . You might be -- but you're not yet -- so stop acting like it.
The "Make Money First" Myth: In the end when you are on your death bed -- will the amount of money you made matter? Or will it be how you impacted people's lives? The time to impact is not after -- it is now and always.
The myth of this graph: (and it's various permutations that are included in most business plans)
There, have I missed offending anyone -- or did I pretty much include them all?
(It should be noted that I have been a personal perpetrator of all of these myths at one time or another in my life. To all those that it negatively impacted -- I am sorry)