Inattentional blindness is an explanation for our inability to see certain changes that are usually considered obvious.
Derren Brown has demonstrated this effect with different races, accents, and even genders! While the original studies had a fail rate of 75%, Brown's informal study had about a 50% fail rate.
Think of all the things we don't notice. Did you notice that in the first video the color of the presenter's shirt changed?
Person Swap at YouTube for Derren Brown's version of this experiment.
What Clown on a Unicycle? Studying Cellphone Distraction at The New York Times for a discussion about a similar phenomenon.