PowerPoint 2011 for Mac does not work well with Apple’s Time Machine backup system. You cannot change the backup timing, so my machine always tries to backup during Israeli office hours (night time US), and guess what, one of the few files that needs a backup is the presentation file I have open right now in PowerPoint. While I am sure there is some sort of file conflict mechanism in place, it is not very effective and I had PowerPoint crashing on me numerous times.
What to do about it? This is a relatively friendly crash, your program is not stuck, it just refuses to save your work (with a frequent autosave setting, that should be about 10 minutes of work). Now here is a nice trick to rescue that chart: select all objects on the page, hit copy, and paste them in an empty Excel sheet. Force quit PowerPoint, re-open, copy the objects in Excel and paste them back in. You just saved yourself from having to redo that 15 layer animation.