We’ve found an example PowerPoint file here http://www.cs.princeton.edu/~sudhakar/talks/memerr.ppt (You will get errors because it includes vertical text and two fonts you may not have installed) that will fail on export back to PowerPoint. However, if you go into the Master Slides and remove the Title & Bullets Master, it will then export successfully.
The problem in this case appears to be the way the Body text is formatted on the Titles & Bullets Master. If you go into the Text Inspector pane, choose the Bullets & Numbering tab, and choose any of the options there, this will reformat the Body and it should export properly. More information will be posted as we discover more about this issue.