If you do any non-trivial customization of Unity Editor, you’ll almost certainly extend EditorWindow. But Unity (to date) refuses to tell anyone how to do this legally, or what the contracts you’re being held to are. Without that, many plugins on the Unity Asset Store are wrong and buggy, and sooner or later accidentally cause problems for users.
Their fault? Partly, but I don’t blame them too much – rather: Unity’s fault for failing to document (and failing to test assets before approving them!).
For those of you who aren’t happy shipping crappy, broken, buggy plugins (And charging for them), here’s a guide / reference for the lifecycle of EditorWindow. Extra info, explanations, and gotchas welcome!