Harut and Marut
Harut and Marut are a pair of angels mentioned in the Qur'an. These angels are send down to earth and teach sorcery. Practicing sorcery is a sin Islam, so some Muslim theologians wondered how angels could do that. Some reject that Harut and Marut are really angels and that they are merely kings (in Arabic kings and angels are simialr words). Others argue that knowing or teaching sorcery isn't a sin. Only praticing it is a sin, like knowing to build an idol isn't a sin, but worshipping an idol is.