How about a Windows Start Menu that is much too "jumpy", which makes it difficult to select a specific item, even when you are trying as hard as you can to click the correct icon? Especially when using multiple-level menus which pop in and out of existence when your mouse moves only a couple of pixels. Considering the problem was so bad that they had to add delays and fades into the system in recent releases just to cover up this irritating, dexterity-dependent system.