Late into this weekend’s ho-hum episode of “Saturday Night Live,” viewers not yet asleep were treated to a great television moment.

The star of this moment was the grungy 20-year-old Ashlee Simpson, the show’s musical guest on Saturday and now the most humiliated pop idol in America.

After host Jude Law introduced her second act of the show, the black-haired Ms. Simpson, more commonly known as blonde Jessica’s younger sister, immediately sensed on stage that something was very, very wrong.

Her band started playing “Shadow,” while studio and television audiences heard a pre-recorded rendition of the words of “Pieces of Me,” the same song she had seemingly sung earlier in the show.

In a matter of seconds, Ms. Simpson’s slithering body went limp and she swung her head back to give a desperate look to the drummer. She was exposed.

As it became obvious that Ms. Simpson had been lip-synching her way through her “Saturday Night Live” appearance, the rising superstar now resembled a red-faced, frozen piano student who just couldn’t finish the recital.

Though the professionals in the band switched over to “Pieces of Me,” Ms. Simpson was down for the count. Appearing to retreat to a distant place in her head, Ms. Simpson kicked her legs in a bizarre jig, fell back into limp mode, hopped a little more, and then shuffled off the stage less than a minute into the song.

The band – acting on an unwritten showbiz rule that mandates never leaving the stage even if your lead singer walks off in a fit of embarrassment – slogged through the song with smirks suggesting, “Hey, don’t look at us.” The show’s producers soon had enough and cut to a commercial.

When she appeared with the cast on stage at the end of the show, Ms. Simpson blamed her band for the mishap.

“My band started playing the wrong song. I didn’t know what to do so I thought I’d do a hoe-down,” she told the audience, the Associated Press reported.

Thirty-year-old “Saturday Night Live” has had its share of bloopers, which are impossible to avoid on live television. They usually involve the cast and take the form of misread cue cards, falling-off costumes, or fits of giggles.

The really memorable unscripted moments tend to involve the invited guests, such as the 1992 episode when Irish singer Sinead O’Connor ripped up a picture of the Pope, or when Martin Lawrence repulsed viewers in 1994 with a raunchy monologue about oral sex.

While the show will likely brush off the latest gaffe, it won’t be so easy for Ms. Simpson, whose path to stardom was laid out by her father, Joe Simpson.

Her “Saturday Night Live” appearance was supposed to be just another step in the right direction. Her debut album “Autobiography,” released last summer, has sold more than 2 million copies. She’s the star of the “Ashlee Simpson Show, ” a hit MTV reality series, and she’s making the predictable shift to Hollywood, getting set to play an actor in the movie “Wannabe,” which is expected to begin shooting next month.

When she stood lifeless on stage, it was clear that no one had prepared her for handling such a moment.

For this young artist, whose raspy singles claim to speak for the millions of vaguely rebellious and confused teenage girls, authenticity is key.

The description of her MTV reality show on the cable station’s Web page says it all: “We see the break-up with her boyfriend and then watch, as it becomes a song on her album. From band practice to label meetings to album cover shoots and everything in between, Ashlee gives us full access to the entire process, taking us through to the album release and the band’s first time ever on-stage performance together.”

Once compared to other young rockers such as Avril Lavigne and Pink, suddenly she’s grouped together with disgraced pop duo Milli Vanilli, who had its 1990 Grammy award stripped after it was disclosed that they didn’t even sing on their own albums. Rob Pilatus, one of the singers for the group, died of a reported drug overdose eight years later.

Yesterday, some of the messages posted on her official homepage were merciless.

“If there was anything more amateurish than your walking off the stage last night on Saturday Night Live, it was blaming the screw-up on your band,” said one visitor to the site. “I am ashamed of you.”

Another wrote: “Fake! All of it. The image, the music, the voice…Stop trying so hard. You’d be more respectable if you would just be what you are.”

And of course there were the “say it ain’t so” fans, who aren’t going to let some television show destroy their hero: “Before you all go off on any of your little jealous raves you should really think twice. Do you know what really happened on the show last night…Ashley (sic) looked beautiful and sang even more beautiful last night.”