forgetI can't believe I've forgotten her birthday!
slip your mindinformalI meant to tell you that he'd phoned, but it completely slipped my mind.
be on the tip of your tongueOh, what was that movie called? It's on the tip of my tongue.
blank outThe dinner where he admitted the affair was so painful that I must have blanked it out.
your mind is a blank/goes blankHe stepped to the mic to sing and his mind went blank.
go in one ear and out the otherinformalHe told me his name, but I'm afraid it went in one ear and out the other.