Alex goes into the mirror and everything is backwards, except her. However, if you look at her carefully, you'll notice that the rips on her jeans are reversed. The horizontal rip goes from the right leg to the left leg, and vice versa for the vertical rip. This is because of an error in how the filmmakers achieved the backwards effect. For the mirror world, Selena Gomez wore a shirt with backwards writing on it, and wore her earrings and bracelets on the opposite hands. Then the film was flipped so she'd appear to be correct and everything else would be backwards. But the costumer designer forgot to make a pair of jeans with reversed rips in them, so when the film was flipped, so were the rips.