The Bradley Effect!
In 1982, LA Mayor Tom Bradley (D) lost the race for the CA governor's mansion even thought he was 10 points up in the polls just prior to election day. The thought is that people will tell a pollster in a race with a white and a non-white canididate that they are voting for the non-white person only to do the opposite on election day. In VA, it is also known as the "Wilder effect" after he won the governor's race by seven points less than on election day. At any rate there have been several articles on this recently and the question is will it have an impact on this year's presidential race. Here is one analysis from Pollster.com and another one from the Washington Post. The other side is the "reverse Bradley effect" found here. Above is a recent clip on the effect from SNL.