X Factor judge Mel B accused one of the show's contestants of cheating during yesterday's live edition.
The former Spice Girl claimed Fleur East's performance was "full of backing track and backing vocals" and she could not hear the singer's voice in parts of the song.
Fleur, 26, performed It's A Shame by Monie Love as part of the ITV show's 80s-theme.
Afterwards, Mel said: "That was a great performance. My only thing is, on that chorus, I didn't hear your voice. It was like it was full of backing track and backing vocals.
"I find that a little bit cheating.
"It was very overpowering."
But Simon Cowell, who is mentoring the over-25s category, was quick to defend Fleur and insisted all of the acts use backing vocals.
He said: "That's really unfair what you said Mel. All of the artists here have backing vocals. You are making the audience think this girl is cheating. It's really unfair.
"We deliberately put the backing vocals up in that track because that's how the track should work. She was singing a different thing to the backing vocals intentionally."
Following Mel's criticism, Fleur told presenter Dermot O'Leary: "Maybe the levels weren't right but I was definitely singing. I worked my butt off this week."
Mel also accused contestant Ben Haenow of being a "wuss" after he complained of feeling ill during rehearsals.
"We all get ill," she said. "When you go on tour, you're just going to have to deal with that. So be a man and don't complain."
Stevie Ritchie's performance of Rick Astley's Never Gonna Give You Up, surrounded by female dancers, led to Louis Walsh calling him "a sex god".
"It was like a really bad dream of being in a karaoke bar in Benidorm," Louis said.
Mel said: "Your personality is adorable. The dancers were hot and they didn't make you look good."
Cheryl Fernandez-Versini said: "You're one of the most genuine contestants I think we've ever had on this programme."
Simon added: "I shouldn't like you but I do like you."
Two of the acts will be voted off the show tonight.
Stephanie Nala and Jake Quickenden were the early favourites to get the boot, according to bookmaker Coral. Stephanie, who sang Blondie's Call Me, was the 11-8 favourite to leave, ahead of Jake at 11-4 after he performed Total Eclipse Of The Heart by Bonnie Tyler.
Meanwhile, Lauren Platt's odds have been slashed to win the show, as she moved just behind bookie's favourite Andrea Faustini.
Seventeen-year-old Lauren sang a version of Irene Carr's What A Feeling, which led to a standing ovation from Cheryl and Mel.
"That was like a vocal masterclass," Louis said.
Simon said: "Hands down the best performance so far for me. Honestly, it was in a different league."
He also praised Cheryl - who he called "Kermit" throughout the show for her striking green dress - for her song choices for Lauren which he said "were not karaoke".