From "Editor" folder EditorWindow scripts can be attached as components to any game object. Attached components can't be removed from inspector.

Expected to see info messages like:
"The script needs to derive from MonoBehaviour" or "This is editor script. To attach a script it needs to be outside the 'Editor' folder".

Pressing play, attached EditorWindow scripts disappear from inspector and console shows error messages:

"Component MonoBehaviour could not be loaded when loading game object. Cleaning up!"

EditorWindow scripts outside "Editor" folder are treated as expected.

Reproducible on 4.5.0a5;