I made friends with this girl, Maria, when it turned out I was in every single one of her classes in the spring semester. Bear in mind that she was only in half of my classes, since I was taking twice the credit hours, but at any rate we got to know each other pretty well and became decent friends. Then, I left for three months with no word and came back as if nothing happened.
Unlike most of my friends, all of whom I was consciously avoiding all summer, I felt bad about ignoring Maria. Not bad enough to call her, mind you, but bad enough that she was one of the first three I got ahold of when I came home.