Enumerator Mistakes

Are enumerator mistakes common in the 1950 census? I just found my father, grandparents, and great-grandparents living in the same house in the Bronx, NY. However, there are two additional children, both aged 1 yr, listed below my father with the same last name. The problem is, no one in our family, who is still alive, knows who these two kids are. Could it be a mistake or have I potentially uncovered a hidden family secret?