The tete-a-tete went down after a performance from the death-defying escape artist Demian Aditya, whose insane stunt was marred by technical difficulties that essentially ruined the trick and left the judges underwhelmed.
"I kind of imagine this would be like Mel B's wedding night," Cowell began when it was his turn to give feedback. "A lot of anticipation, not much promise or delivery."
Before he could get to his punchline, the 42-year-old singer -- who is in the middle of a contentious divorce with her ex, Stephen Belafonte -- stood up and aggressively threw her cup in Cowell's face, soaking his shirt and hair, before storming off stage.
This isn't even the first time this season Mel's thrown her drink at her fellow judge. During the audition rounds, Cowell joked that she'd managed to get famous without knowing how to dance and ended up getting soaked.
"When you lose somebody, you don't really expect to be in the same situation as somebody else when you're on live TV, and they're going through the same thing," she shared. "So when her story came up again, I'm like, 'Oh my gosh, that's very similar to mine.' I wanted to put the story aside and just focus on her voice, but it all just collided. Thank god she has such a beautiful voice."