This website was helpful for me:
http://forums.macrumors.com/archive/index.php/t-101017.html
"as you work. Consequently, if working in Word for a long period of time, and frequently "saving" Word will hit its quota of saves and refuse to save anything else. The error messages ("too many documents open" or "disk full") are bogus and obviously not true in the real sense, but serve to worry the user and displace blame away from ms...
Thus it appears that this bug hits cautious users the hardest - as "saving early and saving often" is precisely what makes it happen. You might say "once burned, twice shy, thrice slapped in the face by microsoft."
The solution when the problem hits is to simply copy your changes since the last save (possibly paste them elsewhere), shut down MS Word, reopen it, then just paste your changes into the document, which will still be saved in its previous incarnation. If editing a document, this can be a bit more annoying as the changes might not be a paragraph or two, but rather a whole pile of little changes spread over an entire paper..." (TheMac19)
Hope that helps anyone in the future!