"The document 'xxx' could not be saved" error

I am using Keynote 2, and had just about finished creating a presentation document with only about 10 slides, but many shape objects (about 25 per slide). Now, when I try to save the document, it gives this error:

The document " filename" could not be saved.

My hard disk has plenty of free space, and I can create and save other Keynote documents. I had been able to save this document up until the last two slides or so -- from then on, I get the error above.

Any thoughts? I don't want to log out or reboot my machine before I am able to save my work. Thanks!