Bill Gates was apparently humbled when he went into a room to watch usability tests and saw a lady trying to use a mouse. She reached the end of the mouse pad,

Bill Gates was apparently humbled when he went into a room to watch usability tests and saw a lady trying to use a mouse. She reached the end of the mouse pad, so she kept moving the mouse onto the rest of the desk surface, and then down the desk leg. It was humbling. Don't know if it's a true story, but can imagine it happening. Never make assumptions about your users ability to think the way you do about your interface. That's unfortunately (and often unintentionally) arrogant and a recipe for disaster.