When you say that something "slipped your mind", it means that you forgot about it. People use "slip (one's) mind" for talking about forgetting to do tasks, or forgetting people's names.
If you didn't remember something at all, you can say that it "totally" or "completely" slipped your mind:
Oh my God. I was supposed to send that off by the 22nd, right? It completely slipped my mind.