Any situation where one or more parties has spontaniously changed their mind or manner, deviating from an understood "persona" or public image or personality. Essencially being hypocritical, two-faced, or sanctimonious.
"She loved me yesterday but today she hats me......Why does she always flip the script on me???"
"You should work 1-2 days a week, thats fine its your schedual, thats a perk of this job."
"You are only ever on 3 days a week, you have to book on five days a week. Your like never here."
To extend your middle finger at the inane dialogue the scriptwriter would have you saying, especially if you are Jack Nicholson.
When I saw my lines for the next scene, I flipped the script.
-- Jack Nicholson