In Keynote '13, A dialog box keeps giving me a warning when I save / autosave The following warning appears:



This happens both when I save and the autosave (which happens often). It is a modal box and interrupts workflow. 

No other applications are running, and the issue survives program and operating system restarts. What could be causing this?

(This issue is new with Mavericks / Keynote '13).
The file might be being modified by another process. You can see as the file is accessed by various processes by running:

sudo opensnoop -f /path/to/file


Use the file normally, then attempt to save. If you get the error, check back in Terminal to see if any other processes have accessed the file.