More than two decades into Lollapalooza's history, and there's no shortage of eyebrow-raising moments. Perry Farrell's formally touring, now Chicago-based fest started as alt-rock's time to shine -- a fact cemented by Rage Against the Machine  and Courtney Love's  presence on this round-up of outrageous Lolla appearances -- but in recent years, pop stars like Lady Gaga  and Kanye West  have brought the shock. In celebration of this weekend's 21st Lollapalooza, Billboard.com presents the top five "WTF?!" moments in the festival's history.
Back when Al Gore's wife Tipper was in the headlines for leading the Parents Music Resource Center, the PMRC raised the ire of music fans everywhere by pushing for the adoption of the "Parental Advisory" sticker on explicit albums and singles. Never ones to abide censorship of any kind, Rage Against the Machine  let it all hang out during their mainstage set at the Philadelphia stop of Lollapalooza '93 when they decided to stand there completely naked, with black tape over their mouths, and the letters PMRC spelled out across their chests, in silent protest of the organization.
Courtney Sings For Kurt
Nirvana  was supposed to be the headlining jewel in Lollapalooza '94's lineup, but on April 7, the band cancelled. One day later, Kurt Cobain's  body was found in Seattle. The grunge act's absence was noted by festival fans from coast to coast, but it wasn't until the Philadelphia date that a surprise visit from Cobain's wife and Hole  frontwoman Courtney Love  gave voice to the void. Taking the stage during the headlining set of her pals the Smashing Pumpkins , Love played a pair of Hole hits, changing the final lyric of "Miss World" to "Why'd you have to die?" and calling for "a moment of silence for my husband."
Lady Gaga Dives In
About four hours before she brought the Monster Ball to Lollapalooza's main stage last year, Lady Gaga  made sure Semi-Precious Weapons  would have one of the weekend's most buzzed-about sets. During the band's afternoon gig on the same stage where she made her Lolla debut in 2007, Gaga surprised the rabid crowd by jumping on the stage, banging the hell out of a cymbal and grinding with lead singer Justin Tranter before diving into arms of squealing fans in the pit. She was the festival's only pop powerhouse, but Gaga gave Lollapalooza 2010 its most punk rock moment.
Nine Inch Nails Trashes The Stage
For a man who would "rather die than give you control," electrical problems on stage in Phoenix during the festival's inaugural year turned Trent Reznor's  rage, literally, against his machines. What better finale for an outdoor July-in-Arizona set full of "Pretty Hate Machine" cuts than for Nine Inch Nails  to smash all of their gear before stalking off the stage in anger?
'Bile Beer' For All!
If you happened venture over to catch the Jim Rose Circus at the Lollapalooza second stage in 1992, you just might have caught the literally gut-wrenching mashup of mainstage rockers like Ministry's  Al Jourgenson, Pearl Jam's  Eddie Vedder  (who also repeated the feat at a '92 JR club show ) or Soundgarden's  Chris Cornell  literally drinking the contents of performer Matt The Tube's stomach. The ew-inducing concoction was quickly dubbed "bile beer." We dare you to try to look away.
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