Continuity mistake: Before Kara uses her heat vision against Vartox she gets some dust on her suit, then she stands up and the dust is gone.
Continuity mistake: When Vartox starts to fight Kara the sun changes between shots.
Continuity mistake: When Vartox attacks Kara from behind the background changes in the next shot.
Continuity mistake: While Kara is fighting Reactron at the junk yard the sun on the floor keeps changing.
Continuity mistake: Livewire has part of her hair over the front of her right shoulder. A shot later after discovering her powers suddenly her hair's over the front of both shoulders.
Continuity mistake: When Alex brings donuts to try to convince Kara to come back to the D.E.O., Kara picks up 2 donuts which suddenly change to 1 once she sits down, and then back to 2 a few shots later in a wide shot.
Continuity mistake: When Cat is on the talk show, the microphone on her collar keeps disappearing and reappearing between shots.
Continuity mistake: While Kara and Alex are on the beach the sun on the floor changes between shots.
Continuity mistake: The bottom circle on the left drawn by Barry on the whiteboard changes shape between shots.
Continuity mistake: While Kara is on the ground after livewire hits her with electricity, the sun changes between shots.
The Adventures of Supergirl - S2-E1
Continuity mistake: When Kara saves the chopper the sun on the floor changes between shots.
Continuity mistake: When J'onn J'onzz is fighting Miss Martian in the ring, she punches him into a wall, leaving a massive dent. She missed the next punch, and the plaster on the right hand side of the dent falls out, but the plaster on the left is fine. Miss Martian then throws J'onn across the room, and in the background, the dent is visible, but it is now the left hand side of the dent where the plastering is missing and the plastering on the right is now mostly in place.
Continuity mistake: Mike and Kara are in the alien bar in the both talking about Mike not controlling his powers. When Mike breaks the glass there is no other glass on the table. In the next shot a glass appears at the edge of the table, in the next shot the glass is gone again. (00:03:30)
Continuity mistake: Maggie comes to Alex's apartment with pizza. When Alex opens and closes the pizza box, she folds the pizza box edges in and 3 shots later the edges are outside of the box.
Continuity mistake: As they are fleeing the alien base, Supergirl and the others run towards the portal, Winn activates it and jumps through. In a following shot (after a 'friendly' alien helps them), he is standing behind Supergirl again. (00:32:00 - 00:32:35)
Homecoming - S2-E14
Continuity mistake: In the scene at the end where Mon-El goes to Kara's to try and console her, right after he sits down and she shifts to lean on him the blanket falls off her shoulder, but in the next shot it's covering her once again. (00:40:35)
Continuity mistake: Rick Malverne has handcuffs on when J'onn goes in as his dad. When he figures out it was J'onn, he tells them they have "23 hours and 14 minutes," looks at his watch and the cuffs are gone. (00:21:30 - 00:25:00)
Continuity mistake: Right after Alex is freed from the water tank, in the shots of Maggie holding her, Alex's clothes are bone dry.
Continuity mistake: As Kenny is about to sit down opposite to Kara, the juice on his tray is lying down on it. Next shot it is standing up.
In Search of Lost Time - S3-E15
Continuity mistake: When Samantha sees the surveillance video of her transformation into Reign, which was triggered by Lena provoking her, their dialogue is slightly, but very distinctly, different than it was a few minutes earlier. (00:20:15 - 00:25:25)
Answer: For one, it's usually cheaper to film in Vancouver because of the tax breaks. But all the CW Arrowverse shows (with the exception of "Black Lighting") are/were filmed in Vancouver. So it makes it easier and cheaper for sharing set designs, crossovers, etc ("Black Lightning" filmed in Vancouver during its crossover). Originally "Supergirl" wasn't part of the Arrowverse when it was on CBS, but it was too expensive to produce and wasn't reaching the wider audience the CW shows had. So CBS moved it to its sister company, The CW.
Bishop73