Continuity mistake: When the truck filled with drums overturns, a drum bounces out on the road. Shot changes and the drum is gone, only to fall again a second later.
Continuity mistake: When Old Barry tries to cross the wall with Young and fails, the way the fallen coat folds on the floor changes between shots.
Continuity mistake: When the Barrys try to flee the lab, Old Barry holds Young's shirt at chest level with his left hand. A shot later, his hand is lowered.
Continuity mistake: When Old Barry collapses in the lab, a yellow bottle lies 20 cm away from him. A shot later it's right next to him.
Continuity mistake: Old Barry collapses next to Young Barry, and lies next to his waist. A shot later he lies next to his head.
Continuity mistake: At the lab, when Young Barry collapses, his head lies on a white tile and his hair is all messed. A shot later it lies on a coloured tile and it's not messed.
Continuity mistake: When the Barrys are at the lab, the headphones are either resting on the table or on top of the coat. This swaps back and forth.
Continuity mistake: While Gary wakes up, the stuff on the table changes positions or disappears between shots.
Continuity mistake: The Flash eats half a burrito, but when it flies towards the knives, it's whole again.
Continuity mistake: When Gary shows his Eric Stoltz tattoo, his arm and hand are resting on his knee. From the opposite angle, they are not.
Continuity mistake: When Barry-1 and 2 fight outside their home, the position of their hands has no continuity at all between shots.
Continuity mistake: When Old Barry meets Young Barry outside their home, the way the underwear folds on Old Barry's face differs between shots.
Continuity mistake: When Young Barry slouches on the chair to eat peanut butter, the way the left side of his robe falls on the chair differs between shots.
Continuity mistake: After Young Barry vanishes, Old Barry holds the robe from the upper part. When Young shows up, Old's hand is nowhere to be seen; the grip has changed.
Continuity mistake: When Young Barry vanishes in the kitchen, the way his empty robe folds changes between shots. Also, the spoon falls on the robe but suddenly appears on the floor a shot later.
Continuity mistake: Barry spits the broccoli and some gets stuck on his lower lip. A shot later it has vanished.
Continuity mistake: Young Barry forces Old Barry into eating broccoli, which he does in large bites. When Young retrieves the broccoli, it's uneaten.
Continuity mistake: When Young Barry enters Old Barry's home, he puts on a robe with a teddy-bear hood. The way the left side of the hood is folded changes between shots.
Continuity mistake: When Bruce is explaining about time travel issues to the Barries, the amount of sauce on the spaghetti changes between shots. The spaghetti in the bowl shifts as well, most noticeably when there's some right up on the lip of the bowl close to Bruce, which then disappears from another angle.
Answer: Some possible reasons gleaned from the Internet: Filmmakers wanted a different vision of Supergirl to reflect a darker, grittier personality and to show her refugee-like experience. Her dark short hair symbolizes her harsher journey from her home world. Also, it gives her a more androgynous look, reflecting both her feminine and masculine traits. Adopting a radically different look for an established character also shakes up viewers' expectations and generates more publicity and interest in the film.
raywest ★