My ancestor has the name Nielsen added before her last name on one of her children's baptism records. On that same child's confirmation record, her last name is ONLY Nielsen. For her children both before and after him, the name Nielson is not used at all in Denmark. I thought maybe it was a mistake on the Baptism record that was copied to the Confirmation record. But then, the name Nielson pops up again on the same child and his brother's marriage records in the United States (but not on their sister's). The family went to the US together. The children were married within a year of each other and the mother was still alive and living in the same town at the time.
Can anyone think of a reason the name might suddenly be added? The other mystery is that I can't find anyone by the name of Niels or Nielsen that the name might have come from. The only thing I can think of is maybe she misremembered her father's first name (she was an illegitimate birth between her mother's marriages to other men). All other records she is using his last name.