This is an unexpected behaviour that has been seen in protected PowerPoint files with a specific patch update of MSOffice 2013.
This happens only if:
Why does this happen?
You are safe if:
1. You haven’t upgraded to the latest 11-Nov-2014 patch for MSOffice 2013.
You should not have a problem if you are using either of these versions or earlier ones.
PowerPoint 2013 - 15.0.4629.1000, Microsoft Office - 15.0.4667.1001
Power Point 2013 - 15.0.4649.1000, Microsoft Office - 15.0.4659.1001
2. You are using an earlier version of PowerPoint like 2010 or 2007.