Continuity: When Supergirl crushes the rock, her hair is curly and messy. In the following shot it's brushed and straight.
Great sites
Mistakes
When Selena breaks the brick wall, the concrete holding it flutters, revealing it's a mere brown piece of fabric. See more...
Supergirl mistakes
Visible crew/equipment: When Selena visits the school, there's a tube reflected on her window. When her car starts to move we see it's part of an umbrella-like structure used for lighting.
Revealing: When Supergirl flies through the window to fight the invisible monster, half of the screen shows her jumping in a school uniform and the other half exiting with the Supergirl outfit. Watch closely and you'll notice that her halves don't match, giving the sense that she is missing part of her body.