"Hey! Could you knock first?!"
When a character is masturbating somewhere such as a bedroom, bathroom, or public space for example and another character walks in on them.
This usually ends with the character that was masturbating to yell at the character or the viewer to go away, look suprised, blushing, or may be too busy having an orgasm from their masturbation to notice someone is seeing them.

See related:

• caught in the act