Too Many Toolbars
As a web development company we're always dealing with the fact that no one's machine is exactly the same. Not just the fact that displays go from 640x480 through 1600xwhatever, but then people install toolbars that take up space.
Here's an extreme example:
I like to believe that this doesn't really happen, but I gotta say, I've had plenty of clients tell me "Hey! I can't see the content without scrolling" and it turns out they're running low-res with a BUNCH of toolbars...