Memory is unreliable because the untrained brain has a crappy filing system. It takes everything that happens to you and throws it all willy-nilly into a big, dark closet. When you go in there looking for something, all you can find are the big, obvious things like when your mom died, or stuff that you don't really need, stuff that you're not looking for, like the words to "Copacabana." You can't find what you need. But don't panic, because it is still there.