You go to an event and see a friend you haven't seen in a long time. However, you look very different than when they last saw you, in fact, you might not even be human anymore. You might have had an emergency transformation to another species, been disfigured badly, or regenerated into a new body. Needless to say, convincing your friend that it's you will be tough, but eventually they get it, and, stunned, say "Is that you?"
(Minor variants of this include characters who've had plastic surgery or changed their appearance to escape the law.)