The singer performs Beyonce's Halo in Mexico for Mel B and guest judge Emma Bunton.
But Mel stops the performance to offer Jack a few honest and harsh words of encouragement.
“Alright, you know what, hold on one second,” she tells him.“You can do so much better than that. You have to look at us, you have to connect with it. You're in you're own world, you're being insulated and you can't do that.
“You've got nothing to lose right now. Just give it, you know what I mean.”
The other boys look on and remark: "What's going on? What's she saying to him?
“This is just awful."
Mel today said she would be a "hard ass bad ass" with her acts.
She told The Sun newspaper: “I can be a hard-ass, a bad-ass, and I can be very emotional and very connected.
“I don’t say, ‘I’m going to put this out today’. It’s just what is needed for that particular person."
Speaking about Jack's performance, Mel added: “For example, I was so angry with Jack that I stopped him and said, ‘What are you doing? Just give me you, what you were giving me before — that’s why you’re here’.”
X Factor airs tonight on ITV from 9PM.
Browse pictures of tonight's show below...