The Dreadlocked One performed the Bob Dylan classic "Mr. Tambourine Man," and forgot the words to the second verse Tuesday.
Wearing a goofy grin afterward, Castro acknowledged: "I lost some lines in there; that's kinda bad."
Judge Simon Cowell thought Castro's gaffe could mean the end of him. "Jason, I'd pack your suitcase," Cowell told the 20-year-old folkster from Rockwall, Texas.
The other two judges were less than impressed, too.
Randy Jackson said Castro wasn't "in the zone." Paula Abdul was, as usual, gentler.
Earlier in the night, Castro sang Bob Marley's "I Shot the Sheriff" and got brutal reviews from Jackson and Cowell.
Castro is not the first "Idol" this season to forget the lyrics.
David Archuleta, who's still in the running, goofed during a Beatles song and castoff Brooke White forgot the words to Andrew Lloyd Webber's "You Must Love Me."