This one that too me a bit to get. There are people out there that need reading glasses but are to vain to get them. So they set the resolution to 600x800. Great the can read it but stuff is off the screen. So when you ask them to click on some thing it's not there. Since you are looking at the screen in normal resolution it does not occur to you to ask the user to scroll. The user is pissed at you cause there is nothing for them to click on.