Have you ever experienced that awkward moment when someone says hello to you, but you can't remember who they are? This happened to me just last week. I was so frustrated because she looked so familiar and she even knew my name -- yet I couldn't remember from where we knew each other. I was experiencing tip-of-the-tongue phenomenon in which I was sure I knew something, but I was unable to access it. This phenomenon can be classified as a retrieval failure in which the information is in there somewhere, but that we can't quite retrieve it.
When I ran into this "friend" of mine, I was embarrassed that I couldn't figure out who she was, that I just politely said hello and nothing else. I was suffering from the memory sin of blocking. It wasn't until hours later that it hit me suddenly that the familiar face I ran into that morning was the girlfriend of a friend of mine who I met while we were studying abroad together in Spain months ago. In remembering this, I felt like a whole set of memories opened up to me again and the next time I see her it won't be such an awkward moment!