Beyond just the obvious "this is a bad way to design things" message, another good takeaway is just the fact that most OO-design tutorials/discussions/books use

Beyond just the obvious "this is a bad way to design things" message, another good takeaway is just the fact that most OO-design tutorials/discussions/books use the worst possible examples that teach people bad habits. When done well, the useful part of OO design is encapsulation and data-hiding, not inheritance, but everyone gets hung up on inheritance because it's easy to cobble together "is-a" examples from real-world-like things, which just trains people to abuse the model. In the real world outside OO-design books and freshman CS classes people don't have separate classes for different shapes, they just have "Polygon" or whatever the graphics buffer requires. And no one has separate subtypes of "AbstractVehicle" for "PassengerCar" and "Motorcycle" and "CommercialTruck," they just have a "Vehicle" class with a bunch of data on it and rely on the client code knowing what to do with that data.