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)
Shelter from the Storm - S3-E18
Continuity mistake: When Winn and Brainy hug, Winn starts the hug with one arm high and one lower, towards Brainy's waist. The shot cuts and both his arms are up and in the centre of his body.
Continuity mistake: The shield design that Winn sketches on the paper changes between him starting to draw, and him showing it to his friend a second later.
Continuity mistake: The sketch that Brainy shows to Winn is meant to be his original sketch that he drew in the previous episode, hence why Winn is so shocked he's somehow got it. But it's slightly different from what we actually saw him draw at the time.
Answer: Another part of why the change was that it's cold in Canada in the winter and the old costume didn't protect her from it, whereas the new one is nice and warm.