Acting like a Requirements Monkey.
Punting on strategy.
Not focusing on a particular target market.
Not meeting with enough customers often enough.
Not meeting with prospects and non-customers (aka potentials) often enough.
Not truly understanding the real problems faced by your target market.
Hearing only what you want to hear.
Being afraid to draw pictures, lest you offend your UX designer.
Writing a Magnum Opus of a requirements doc or strategy doc, primarily to cover your ass.
Forgetting to incorporate features into your product that help you measure success or failure, and thereby improve over time.
Going along with a development process that can't adjust when faced with negative market feedback.
Becoming Development's co-dependent, and having them come to you about the placement of every freakin' pixel.
Allowing a piece of junk to ship.
Making the product hard to buy or up-edition.
Thinking that landing reference customers for a new product/release is someone else's job.
Letting the release treadmill create a "boat anchor" editioning and pricing situation, where customers can't upgrade without major migration headaches or losing functionality.
Assuming that everyone that stands in your way is an asshole or a political player.
Neglecting to spend the time building rapport and credibility with engineers.