Toscano's surprising elimination after the Rock and Roll Hall of Fame-themed week mentored by will.i.am left eight finalists fighting it out to win season 10 of the hit TV talent show.
A clearly upset judge Jennifer Lopez buried her face in her hands and said "I have no idea what just happened here. I'm shocked. I'm angry."
Veteran judge Randy Jackson echoed her words, saying "This makes me mad". He told Toscano, who switched from her usual ballads to an up-tempo performance this week, "You're one of the best singers in here."
Toscano, 22, from Queens, New York City, went home to a standing ovation from the judges followed by a performance of "I'll Stand by You."
Joining Toscano in the bottom three were Stefano Langone and Jacob Lusk, both of whom were pegged by bloggers as a good bet for elimination this week.
The judges and host Ryan Seacrest all exhorted fans of the show to make sure they voted for their favorites, theorizing that perhaps Toscano was booted because her fans assumed she was safe from elimination.
After bringing in Lopez and second new judge, rocker Steven Tyler, the nation's top-rated TV show has recovered this season from several years of flagging ratings, holding steady at about 25 million viewers.
Thursday's show featured a performance of "Unchained Melody" by season four alumnus Constantine Maroulis, as well Iggy Pop, characteristically bare-chested, who sang "Real Wild Child."
The other remaining finalists include Casey Abrams, who last month scored the judges' once-per-season save, Haley Reinhart, James Durbin, Scotty McCreery, Lauren Alaina and Paul McDonald.
"American Idol"'s finale airs on the Fox network in May when viewers will choose the next "Idol" star, and he or she lands a recording contract.
Past winners include Grammy-winning artists such as Kelly Clarkson and Carrie Underwood, while even "losing" contestants such as Clay Aiken, Oscar winner Jennifer Hudson and Adam Lambert have become breakout stars in their own right.