Possible to over-knead dough?
Cookbooks describe the state as "smooth and elastic" I think this is a reasonable description.

When the dough is first mixed it is very wet and sticky. As it is mixed you can see a lot of clumps and heterogeneous textures.

As the proteins in the flour mix with water they form gluten and the kneading folds the elastic gluten over itself again and again making sheets. The dough becomes less sticky and wet and more springy. The texture becomes completely homogeneous.

Kneading by hand it is impossible to over knead this type of dough. I have read that it is possible to over knead in a machine. In this case, supposedly, the protein sheets eventually rip up enough that you lost what you created and the dough reverts. I have kneaded in a machine for a long time and never personally seen this effect.