@kegibson
Thank you for sharing this log file with us. I have kicked it to the dev team to assess.
Q1: does the file save problem happen often? For example is it only for a single design file (.mobi file) or does it happen for multiple design files?
Q2: Can you tell me a little about your computer? For example what Windows OS, (XP, Vista, Win7) and how much RAM does the machine have?