example.cs

class Rectangle
{
    protected double Width = 0;
    protected double Height = 0;

    public Drawable Render(double area)
    {
        // ...
    }

    public void SetWidth(double width)
    {
        Width = width;
    }

    public void SetHeight(double height)
    {
        Height = height;
    }

    public double GetArea()
    {
        return Width * Height;
    }
}

class Square : Rectangle
{
    public double SetWidth(double width)
    {
        Width = Height = width;
    }

    public double SetHeight(double height)
    {
        Width = Height = height;
    }
}

Drawable RenderLargeRectangles(Rectangle rectangles)
{
    foreach (rectangle in rectangles)
    {
        rectangle.SetWidth(4);
        rectangle.SetHeight(5);
        var area = rectangle.GetArea(); // BAD: Will return 25 for Square. Should be 20.
        rectangle.Render(area);
    }
}

var rectangles = new[] { new Rectangle(), new Rectangle(), new Square() };
RenderLargeRectangles(rectangles);