The former "Mad Men" star portrays a teacher with a dark personality in the movie, and one scene required her to show off her secret knife skills to a fellow staff member, as played by Ice Cube.
Christina expected to pull off the move with a small folding blade, but she was left panicking when she realised the prop producers actually had in mind was a lot bigger than she had imagined, reported Entertainment Weekly.
"I had bruised fingers because I knew I was supposed to have a knife, but I thought it'd be a switchblade," she said on "Today."