I bumped into a subtle conflict this evening. I created a new file from a stock template. I then used Insert from File > Insert Views from File to acquire a few drafting views. When I closed this new project and decided to open the file the harvested drafting views are stored in this message appeared.
Keep in mind that no files were actually open at the moment. I was looking at the Recent Files list yet when I attempted to create a new local file for the project I just used Insert From File on the message popped up. This means that the file is technically still open in RAM as far as Revit is concerned, it's just not open for me to interact with.
I had to exit Revit so it could relinquish its hold on the file before I could start Revit up again to get back to work.