Mariah Carey Was A 'Nightmare To Work With' On American Idol

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.