In episode 1, "Smith and Jones", there is a scene towards the end, where the Plasmavore drinks the Doctor's blood. She does this by sticking a straw into the right side of his neck. When Martha revives him, you see that side of his neck and you would expect there to be a small circular cut or something on his neck, where the straw went in, but there isn't.

Note: Feel free to correct me, I just noticed this the other day.