In the scene where Blaine is asking Andy out.. She is sitting outside in the "outcast" section of the school I guess.

Andy is sitting on a bench, and Blaine is next to her. When the focus is on Andy, you can see behind her a girl wearing weird leggings and a weird skirt. Then, the focus jumps to Blaine, and the same girl is behind HIM, and so on as the focus switches between the two actors.

It bugs me each time I watch it.. ;)