Thanks for the response. To answer your question, the instance in question does not show up in any location in the ODK main menu -- edit saved form, send finalized form, or delete saved form. The only location where I see the instance is in the odk\instances\ folder on the device file system. In this case, it's just an empty folder.
So the count of folders in odk\instances\ is one more than the count given in the menu at "Edit saved form." It's as if it doesn't recognize it.
I suspect that they might have started a new form, but maybe tried to exit without filling anything out and hit "Ignore changes." I'll try to reproduce this scenario.