Go to the part just after Callie tells Mark that she won't sleep with him and Alex hears. Derek goes running up the stairs just as Meredith goes to run down them. They stand there for a second, like its kind of awkward. The camera right now is facing Derek. Meredith reaches up and scratches her shoulder. Right when she's got her hand at her shoulder, the shot cuts to face mostly her. And she is not scratching her shoulder and her hand is down by her side. It's pretty obvious, not very hard to miss. So you should be able to catch it easily.