When I went to vote in the primaries, there were black people who were the helpers and one saw that I was white (and therefore was probably not voting for Obama, although he had no way of knowing) and he pretended that he couldn't find my name in the book of voters. I had to point my finger right at it before he'd acknowledge I was there and then he scowled a little and gave me my little ticket so I could use the voting machines.
Be very careful of shenanigans like this when you go to vote. Make sure that if someone says your name isn't in the book that you look yourself, and call over someone overseeing the election process if you have any doubts that everything is going correctly.