Factual error: Supergirl spots Selina on school grounds and is about to leave the classroom to investigate, but Nigel, the teacher, stops her. In the background, you can see a small room with beige computers. These are BBC Model B computers, a UK computer that was typically used in UK schools and wasn't sold or available in the US, showing that the school featured in the movie is actually in the UK and not the US as depicted.
Supergirl (1984)
1 factual error - chronological order
Directed by: Jeannot Szwarc
Starring: Faye Dunaway, Peter O'Toole, Mia Farrow, Hart Bochner, Helen Slater
Continuity mistake: At the beginning of the movie, Selena drives her car to the right side of the house. When the shot changes, the car is several feet further back and is now heading across the front of the entrance.
More questions & answers from Supergirl
Answer: It's never explained, she could have changed at super speed. In Superman II, when confronted by the super criminals at the Fortress of Solitude, Superman did a disappearing and reappearing act. So did the criminals. It might be a similar thing.