What are some good examples showing that "I am not the user"?

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.

I think one of the biggest examples is that expert users tend to play with an application.

They say, "Okay, I have this tool, what can I do with it?"

Your average user sees the ecosystem of an operating system, filesystem, or application as a big scary place where they are likely to get lost and never return.

For them, everything they want to do on a computer is task-based.

• "How do I burn a DVD?"
• "How do I upload a photo from my camera to this website."
• "How do I send my mom a song?"

They want a starting point, a reproducible work flow, and they want to do that every time they have to perform the task. They don't care about streamlining the process or finding the best way to do it, they just want one reproducible way to do it.

In building web applications, I long since learned to make the start page of my application something separate from the menus with task-based links to the main things the application did in a really big font. For the average user, this increased usability hugely.

So remember this: users don't want to "use your application", they want to get something specific done.