Mos Def whom is now Yassin Bey has been denied entry into the USA. Weird, because he is an American? Well, due to his public statements against certain treatment of prisoners in Guantanamo Bay, the USA deemed him a national threat. He had an upcoming tour scheduled and this  was the statement released:

"We regret to inform you that due to immigration/legal issues Yasiin Bey is unable to enter back into the United States and his upcoming tour has been canceled."

Mos Def has lashed out against various unfair treatments and ironically has touched on immigration issues as well.