In Word, you can have multiple windows open for the same document. Go to the menu along the top, click Window > New Window. In this way, you can have two pages of the same document open at the same time. It’s great for editing. Good feature. (Thanks, Microsoft!)
What’s bad is that if you try to insert a citation while having two windows of the same document open, the citation will be added to the first window of the document that was opened, regardless of which window has focus. This can be confusing.
Also, if you try to add a citation in Word using Endnote while in full-screen mode, it won’t let you, because it thinks you’re in Publisher Mode.