Complex sentence: These mistakes may have been a result of nerves (it was rare to have a black man testifiying against a white man in court during this time period) or the fact that Powell was seated at the moment of the reference.
Split into simpler sentences: These mistakes may have been a result of nerves as it was rare to have a African American testifying against a white man in court during this time period. It may have also been the fact that Powell was seated at the moment of the reference.