NSDocument: The document cannot be autosaved. The file was modified by another application.

A search in the title of this post indicates that it is fairly common; indeed, I got this error from Xcode. But I can not find any fixes. I see this now when I run my program, and it appears during or after the call to changeCountTokenForSaveOperation. This seems to be related to the cancellation manager, not the fact that I am using basic data, but I could be wrong.

Does anyone know what causes this and how to fix it?