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I'm a software developer who has a background in usability engineering. When I studied usability engineering in grad school, one of the professors had a mantra: "You are not the user". The idea was that we need to base UI design on actual user research rather than our own ideas as to how the UI should work.
Since then I've seen some good examples that seem to prove that I'm not the user.
- User trying to use an e-mail template authoring tool, and gets stuck trying to enter the pipe (|) character. Problem turns out to be that the pipe on the keyboard has a space in the middle.
- In a web app, user doesn't see content below the fold. Not unusual. We tell her to scroll down. She has no idea what we're talking about and is not familiar with the scroll thumb.
- I'm listening in on a tech support call. Rep tells the user to close the browser. In the background I hear the Windows shutdown jingle.
What are some other good examples of this?
EDIT: To clarify, I'm looking for examples where developers make assumptions that turn out to be horribly false about what users will know, understand, etc.