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.
Muhammad's companions say that Harut and Marut have been angels in heaven who were outraged by human's sin. God then made a bet with the angels, that they wouldn't be better, if they lived on earth with human urges. They agreed but actually sinned on earth and so they were not allowed to return to heaven. Further, they have to choose their punishment, either in this world or in hell. They choose earth, and so they became (fallen) angels tempting people into sorcery.