Toasters

Inattentional blindness. [Link]

People can't see what is right in front of their face. Even though there's a warning right above the disabled item explaining that it can't be changed due to an administrative setting, even though there's a link on the page whose text directly addresses the issue they are asking about, even though they confirm the existence of the explanatory text when asked, they still don't read it or understand it. It's another case of inattentional blindness. You are so focused on the task of clicking on the combobox that you completely disregard any other information because you simply aren't expecting it to be there.

I think this is mostly correct. In my experience, people most computer users (not your advanced users, developers or admins) memorize steps to complete their tasks. That's all. They are not interested in learning new ways of doing things that are easier unless it is radically easier to do. They learn just enough and thats all. As soon as something pops up that breaks that chain of steps, they are boggled. An unexpected messagebox, a disabled control, whatever.

Most people don't like computers or using computers, they are just a means to an end. We need to start designing things more for them than for us. We make toasters that have umpteen different conflicting, interacting settings, that breaks down regularly. We can specify bread type and thickness, the temperature of the coils, schedule for later toasting, watch the toasting in progress, view the power consumption over time of the toaster. They just want to make a piece of toast, medium.