No. 1: There is IT and there is business
Binary Thinking: Everything that exists within an organisation (strategies, policies, processes, people, teams, assets, budgets, requirements, projects etc) can have a label of "IT" or "Business". Nothing can carry both labels. Organisations need to work hard to make sure all of the "IT" things and "business" things are aligned but merging any of these things would be a mistake. Usually "business" people prevent "IT" people from participating in "business" activities. If "business" people work with "IT" suppliers it is bound to cause problems.
Portfolio Insight: Everything in the organisation is about the business, a subset of these things have specialist IT elements. For example, a specially trained engineer may help maintain some computer equipment but this is a small component of an internal service which keeps the organisation running. It is helpful to break down large teams, systems or change programmes into smaller components to make them more manageable but very often there are much more useful groupings to use than IT and non-IT.
Special Marketing Hex: A new variant of this curse is the confusion surrounding IT spending by the CMO. Most of the posts about this subject suffer from Binary Thinking by insisting that a CIO cannot possibly have any influence over spending which is labelled as "marketing". Thankfully some, like Chris Murphy of Information Week, are willing to challenge. His blog can be found here (with thanks to @ij_cox for sharing it).
Binary Thinking Hex No. 2
Related blog post: Will CMOs Outspend CIOs? Wrong Question