In the Rebellion film, we see Homura turn into what she realises is a witch (after she discovers some oddities about the version of Mitakihara city she's supposedly living in) because of her despair.
This reason for her turning into a witch makes sense to me because it would be "valid" under the laws of the universe prior to Madoka's changing of them. However, I'm a bit confused about how Homura managed to become a witch after the law of cycles came into "effect", if under this regime, magical girls disappear instead of becoming witches once their energy is used up.
Why exactly then was there this "exception" to the law of cycles? (I might have missed something since I haven't really followed Puella Magi Madoka Magica very closely.) I suppose the fact that Homura is the only person around who knows of what the former "regime" was might be important in this, but I think I probably missed something when I watched the film.