Continuity mistake: At Selena's party, while Nigel is talking to the blonde girl, the green straw in his cocktail moves around between shots.
Continuity mistake: At the end, when Ethan is saying goodbye, in the wide shot there's barely no one behind the red car on the left. In the close shot a lot of people appear.
Continuity mistake: Supergirl comes out of the lake but her hair is dry.
Continuity mistake: When Supergirl and the principal enter the girls' dorm, the jogger girl turns around to run away twice.
Other mistake: The very last scene of the movie shows Supergirl flying underwater holding the Omegadron. Oddly enough, though her body is realistic, her arms are now very thin and measure two meters long.
Revealing mistake: In the final confrontation, when the force of shadows pulls Supergirl forward her body begins to tilt backward as she struggles. As she does, you can see the straps of her flying harness begin to peek out from underneath her super skirt. (01:53:40)
Continuity mistake: In the castle, when Supergirl asks Selena for the Omegaheadron, Selena turns to her left and opens her eyes widely. In the following shot she repeats the same movement and expression.
Continuity mistake: When Selena makes a toast with Ethan, from the front the logo and name of the beer is visible, in the following shot it's not.
Visible crew/equipment: When Supergirl first starts "flying", her movements easily reveal the use of wires to give the effect.
Revealing mistake: When Supergirl is flying at super speed, the cars from the green-screen movie are also sped up.
Continuity mistake: When the truck crushes the red car, the woman with the trolley makes a right to protect herself. When the angle changes she repeats the previous movement.
Deliberate mistake: When Supergirl flies out of the lake upon her arrival to Earth she's wet. When she lands on the shore she's suddenly dry.
Suggested correction: Incorrect. She's not wet when she comes out of the water. She emerges from the water dry in an (incredibly wonky) effects shot. Why she's dry coming out of the water is unclear, but it presumably has something to do with the teleportation process for how she got to Earth considering her costume also suddenly appears on her as well.
Plot hole: Superman (or Kal-El) was a baby when his parents sent him away from Krypton before its destruction, and as Jor-El told his son in his first message he would have already been dead by then for thousands of Superman's years. Kara never existed during any of this yet so somehow she instantly recognizes him from the poster on the wall, even though she should have no idea who he is.
Continuity mistake: When the movie ends, Supergirl dives into the water, then the immediate submarine angle shows her completely dry. WTF?
Visible crew/equipment: During the tractor scene, a wiggling boom mike is reflected on the bar's mirror.
Continuity mistake: After Lucy leaves out for the weekend, while Supergirl is sitting on the grass, the people standing by the red car behind disappear between one shot and the other.
Continuity mistake: When Ethan falls unconscious on the couch, his arm is close to his face. In the following shot it has moved down.
Continuity mistake: When Ethan enters Selena's house, Selena opens the fridge. Her position behind the door changes from being in the middle to the right of it between shots.
Continuity mistake: When Supergirl visits the principal's office, he is holding the glass down, close to the medicine bottle. When the angle changes, the glass is up without him having raised it.
Continuity mistake: When Supergirl wakes up in the woods, the baseball's stitches face different sides depending on the angle.
