Mariah Carey was a "nightmare to work with" during her brief stint as a guest mentor on "American Idol" in 2008 -- this according to sources on the show.
Sources tell TMZ, Mariah -- who made her mentor cameo on "Idol" Season 7 -- was incredibly late for her shoot at the Beverly Hills Hotel," not very nice at all," and ridiculously nitpicky about tiny details.
In contrast, the same sources agree J.Lo was a pleasure -- "night and day" compared to Mariah. We're told J.Lo was all about business, in and out, no mess, no fuss.
As we reported, Mariah has been tapped as the newest judge for "Idol" next season. She signed a deal worth roughly $18 million for her first year on the show.