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Stevie Nicks, season 11 (top 9, Songs of Their Idols)
Stevie was warm and gracious during her sessions with all the contestants, but she had a soft spot for Phillip Phillips: She said Mick Fleetwood would have chosen him for Fleetwood Mac if they had been friends back in 1975. Heejun Han told reporters after he was eliminated that she was his favorite mentor and that she came well prepared for her sessions with the Idols.
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Lady Gaga, season 10 (top 4, Songs that Inspire/Leiber & Stoller Songbook)
Gaga made quite the splash when she mentored the remaining four season 10 Idols. She arrived in a pair of heels shaped like, um, a specific male body part. After she instructed a mortified Scotty McCreery to handle his microphone like a girl who was demanding he stick his tongue down her throat, Scotty kissed his cross. ''Lord, this is not my doing,'' Scotty joked.
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Miley Cyrus, season 9 (top 11, Billboard Hits)
When it was announced that Disney star Miley Cyrus would be mentoring the Idols, eyebrows went up all over Idol land. But it turned out that the teen star, who began her show business career at the age of 11, had some helpful performance tips.
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Adam Lambert, season 9 (top 9, Elvis Presley)
Who better to mentor the finalists than someone who has been there and done that? Adam was the first alum to return to Idol as a mentor. His advice to the finalists was honest but constructive, and concentrated on how to craft a performance that is vocally, visually, and emotionally satisfying.
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Harry Connick Jr., season 9 (top 5, Frank Sinatra)
The legendary crooner not only employed his impressive musical skills during his mentoring gig by arranging all the songs and leading the band on stage, but he was easily the funniest mentor Idol has ever had, disarming the nervous finalists with his easy charm.
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Quentin Tarantino, season 8 (top 7, Songs of the Cinema)
Quentin Tarantino is best known in Idol circles for his season 3 guest judging stint when he told a stunned Diana DeGarmo, ''You pulled off the notes and stuff, but I don't care'' after she delivered a particularly pageant-esque performance. The Kill Bill director returned as a mentor five years later, kinder and gentler, but still eccentric.
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Slash, season 8 (top 4, Rock Music)
Rock star Slash not only mentored the Idol finalists but he rehearsed with them at the Roxy in Los Angeles. And no, the ex-Guns N' Roses guitarist did not instruct Danny Gokey to caterwaul like a dying cat during the last note of Aerosmith's ''Dream On.''
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Mariah Carey, season 7 (top 7, Mariah Carey Week)
Fans assumed the guys would be at a disadvantage when required to sing Mariah Carey songs. But instead, with Mimi's help, the boys stepped up to the plate and delivered — particularly David Cook, who performed a heartfelt and stunning ''Always Be My Baby.''
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Jon Bon Jovi, season 6 (top 6, Bon Jovi)
The affable Jon Bon Jovi was kind and helpful, but very protective of his own work. When Blake Lewis presented a radical reworking of the band's classic hit ''You Give Love a Bad Name,'' he was skeptical. But Jon rolled the dice along with Blake, and the result was a performance that became one of Idol's greatest musical moments.
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Queen, season 5 (top 8, Queen)
The finalists not only had the opportunity to sing the iconic songs of Queen, but they got to rehearse the tunes with the legendary band. Maybe it wasn't so great for Ace Young. When he presented his own twist of ''We Will Rock You,'' guitarist Brian May would have none of it. ''I don't want you to do that to my song,'' he said bluntly. Later, Brian would blog that the incident was blown out of proportion, but the damage was done. Ace would forevermore be known as the guy who disrespected Queen.