designmuseum.org : A case of Design before Usability ?
Confusability — “I was trying to use my arrow keys on my keyboard to navigate the flash version and when I switched to the HTML (reduced content) version I found it to be unusable if you have popups blocked.”
Most sites don’t design for failure, or change. They expect linear user activity, but that is not realistic. You generally need at least some level of failure control, consistency, and redundancy, to confront this in a design.