Finger memory…
Have you ever caught yourself trying to remember a password when asked and then being not able to answer? I did. Not only once. And the strangest thing is that I simply do not remember the password. I have typed it so many times that at the end it sits in my fingers. The mind gets rid of unnecessary information counting many of those passwords as such. And we have it there: I can logon into many web-services correctly typing something that I do not remember. This is the best kind of security one can get isn’t it :-)?
There is surely a research done in this field so if you know an interesting link, please drop me a line in the comments.