CKEditor can be loaded before registering external plugins.
When this is the case, plugins are loaded from the default lookup path for CKEditor plugins, i.e. relative to ckeditor.js, while there is no such plugin there.
Hence this kind of warning in console
This typically does not happen at first load because by the time the CKEditor library is loaded, first updateFromUIDL has already completed, and populated the list of external plugins.
On subsequent occasions, it is not granted that external plugins are populated before loading the editor.