Used when someone’s behaviour is well out of line and you’re not laughing about it. Out of order is the base, and bang just cranks it up to maximum, like a verbal yellow card turning red. Say it for dodgy, unfair, or disrespectful moves when you want them to know they’ve messed up and need to fix it.
"He jumped the queue at the chippy then acted innocent. That’s bang out of order, mate, get to the back and stop being a melt."