1. You have met the person, but do not remember having met them, or do not remember their name. This version of retroducing usually leads to an awkward, detailed retelling of how you met, while you listen and feel like a jackass.
2. You have met the person, but you do no think that they remember you, so you introduce yourself again. You are forced to decide how to explain yourself, whether you would have them think that you are absentminded, rude, or that you consider yourself to be completely forgettable.