I'm working on some Italian records and ran across this weird situation. Why would a mother's surname change from record to record? Here's the FamilySearch profile I'm looking at -- the woman's name changes between Bellitti and Ciaffo almost at random over the course of two decades, including one case where the birth record for a child used Bellitti, but the death record seven days later used Ciaffo. It's too deliberate to be a mistake.
Bellitti does seem to be the family name -- her parents appear on her marriage record, and I don't see anything indicating she was adopted, and no clue as to where the name Ciaffo came from. And unfortunately I can't get to the actual records themselves to see if this is some sort of transcription error.
I've seen a couple unusual examples where somebody starts using a different name -- usually an orphan, but I don't think I've seen another instance where they bounce back and forth between names.