Back in 2002 there was a movie that came out titled "Catch Me If You Can" which was a story about how events in Frank Abagnale's life lead him to eventually becoming one of the world's well known con artists. During his jaunt, he successfully conned millions of dollars worth of checks. He also was able to pretend to be a Pan Am Pilot, a Doctor, and a Legal Prosecutor.
One of the things that lead to this was a lesson that his dad taught him early in the movie. The conversation went like this...
Frank's Dad: When I get inside, you go back to the front seat and wait. Even if a cop comes and writes you a ticket you don't move the car, understood?
Frank: "Dad, wha-what's all this for?"
Frank's Dad: "You know why the Yankees always win, Frank?"
Frank: "Cause they have Mickey Mantle?"
Frank's Dad: "No, it's 'cause the other teams can't stop staring at those damn pinstripes."
Following this conversation, the dad went on to demonstrate to his son how to con someone at a bank into getting Frank set up with his first checking account when he then we on to write bad checks with.
This "staring at those damn pinstripes" is what we like to refer to as "The Halo Effect" which is a phrase coined by Edward Thorndike back in the 1920's. He was the first to support this phenomenon with empirical research (which we say "Bravo!" to).
The halo effect is when an observing person subjectively and assumptively develops an unfair bias towards an observed person. This bias is generally based on some characteristic of the observed person such as the shape of their body, their clothing, the persons personality, etc. In the example above, Frank's Dad is suggesting that the teams that play against the Yankees are so busy staring at the stripes on the Yankee's outfits that they become detracted from playing their best.
What's this got to do with UX? This "Halo Effect" is one such example of cognitive understanding that the User Experience Practitioner must consider in their planning, molding, and maintenance of the interfaces they come in contact with.
It is for the reason of the "Halo Effect" that it is a good idea to test while a site is still a wire frame. When a design has been advanced to higher fidelity, users will likely become distracted by the attractiveness of the design. In fact, it is likely that if asked, they will say that the design is great. The problem with this is that even thought the visual design may be great, the design's affordance for usability may still be poor. By testing in the low detail phase, a user will be less likely to be caught "staring at those damn pinstripes."
Halo Effect, Wikipedia
What is the Halo Effect?, About.com Psychology
Catch Me If You Can, Wikipedia
Rapid Desirability Testing: A Case Study, Michael Hawley, UX Matters