Continuing with the coal theme: here's an expression that received a lot of (unwanted???) attention when Massachusetts Governor Mitt Romney used it to describe a construction project in 2006 but was "hauled over the coals" for what is considered a racial epithet by some.
The tar baby is supposedly a popular character from African folklore. It gained popularity in the 19th century United States in the written form in one of Joel Chandler Harris's Uncle Remus stories, a collection of stories based on African-American folklore, narrated by the fictional Uncle Remus, a former slave. In the story, "Tar Baby" is a doll made of turpentine and tar, built by Brer Fox to entrap his enemy, a tricky and cunning Brer Rabbit. Brer Rabbit talks to the doll, and when it doesn't answer, he hits it, and gets stuck in the tar. The more he struggles with it, the more he is entangled in it.
It sounds like yet another harmless story, doesn't it? For reasons better known to this hard-to-comprehend world, (or perhaps I am missing some subtle nuances here???) the expression has been occasionally used as a derogatory term for African-Americans. Needless to add, public figures who choose to use it encounter controversy.
Source: http://www.randomhouse.com/, Wikipedia
Image: Wikipedia (Br'er Rabbit and the Tar-Baby, drawing by E.W. Kemble from The Tar-Baby, Joel Harris, 1904)