Pages: Permission errors when writing documents

Calvin here, from time to time while working in Pages, I will go to make a change and suddenly I get a drop-down window that says, "You don't own the file XXXXXXX and don't have permission to write to it." This particular message I had the document locked, and I unlocked it. Other times I've gotten a similar message saying I don't have permission to make changes. I have never required a password to get into any of my files.

What is going on?