// Un campo que debía ser de cada objeto se declaró static "para que compile".
public class StaticPitfall {
    static class Tally {
        private static int count;          // error: un solo contador para todos

        void click() {
            count++;
        }

        int count() {
            return count;
        }
    }

    public static void main(String[] args) {
        Tally door = new Tally();
        Tally window = new Tally();
        door.click();
        door.click();
        window.click();
        IO.println("door: " + door.count() + ", window: " + window.count());
    }
}