Jessica Alba makes fatal faux pas - if you screw up in front of Ashton Kutcher he WILL let the whole world know about it.

Alba was more than red-faced lensing a lip-lock with Ashton for new rom-com Valentine's Day - her chewing gum nearly fell out of her mouth during the steamy scene.

"Whenever I'm nervous I have gum on the roof of my mouth," Jessica divulged.

"So I had it (during the scene) and my mouth got dry because I was nervous because I knew I was going to kiss Ashton.

"It got powdery and it got on my lip during my close-up."

Needless to say, Ash spotted the gooey protuberance.

"He called me out in front of the whole crew," Alba said.

"It was really embarrassing."