Terrence dials a number on his cell phone. His friend, Will, answers.
Terrence: Will? Where are you?
Will: At home. What are you doing, Terrence?
Terrence: Waiting for you! We agreed to meet in Downtown, remember?
Will: I'm so sorry, Will. It totally slipped my mind. I'm on my way!
Linda and John arrive home to find dinner in the oven and it's burnt.
Linda: John, weren't you supposed to turn the oven off before we left?
John: I guess it slipped my mind.
Meaning: to forget. We use this idiom when we have a thought or intention and that thought or intention "slips away" before it becomes an action.