Continuity mistake: In the scene on the roof after the power transfer, Hannah falls through the sky light and Matt yells down to her. You can tell the close-ups on Matt were re-shot because Matt's hair is longer and styled differently in between shots.
Continuity mistake: When Matt is chasing the street robber at the start of the movie, the robber is out of sight of Matt when he drops G-Girls purse under the silver car, yet Matt hasn't seen this and cannot see the purse yet knows where to pick it up.
Continuity mistake: During the last few scenes in the movie, when Hannah asks Jenny if she needs help with the plane, they knock knuckles together. When the shot is taken from behind Hannah, she is shown wearing gloves, but when the shot position changes to behind Jenny, the gloves magically disappear.
Factual error: In the opening NYC MTA subway scene a couple minutes into the movie, the characters are riding an R subway train. The letter is visible behind Luke Wilson's head when he asks, "Wouldn't it seem a little - yeah". When Luke Wilson runs off the train after a robber a minute or two later, they run up a staircase that says "uptown N R W trains". When they run out of the station, however, they are running out of the Franklin Street 1 stop. The red 1 line and the yellow N R W line do not intersect anywhere near Franklin Street, so it would be impossible for them to have exited an R train at the Franklin Street station.
Continuity mistake: In the scene at Professor Bedlam's mansion lair where he demonstrates the strength of G-Girl's hair, when the flying buzz-saw blade finally part-buries itself into the chest of the larger of the two goons, it's at an angle that points towards his chin, but as he falls back onto the chair, the angle has suddenly changed by around 90°.
Other mistake: When Jenny and Matt first enter his apartment, the deadbolt on the door is intact. Later, when Professor Bedlam's minion kicks the door open the plate and deadlatch are missing. The act of kicking the door open might destroy the striker plate in the door jamb, or it might blow the entire lock body through the door, but it couldn't neatly remove the plate and bolt alone.
Continuity mistake: In the scene where Jenny Johnson escorts Matt Saunders back to her apartment intending to reveal her true identity as G-Girl, upon entering, Matt takes off his jacket and places the vast majority of the jacket over the arm of a chair. Jenny eventually guides him to be seated in that chair. The jacket is repositioned to being less over the arm and now it is in the center of the chair, but just before Matt sits down the jacket is repositioned again to be more to his right. (00:31:05 - 00:32:10)