I suspect you have in the past used Microsoft Word or similar to create product descriptions and paste them in. This page for example (
http://www.monstersinmotion.com/cart...ion-cd-p-18058) has a ton of invalid characters such as ’. So if you were to edit this page in your admin it would likely disappear as well if you re-save it.
The problem is when a character, code, javascript or similar is in the product description your admin cannot display the description. It's there, but to erred to be displayed. If you save it "eats" it if you don't it remains.
We have seen this a great deal.
The fix is to copy and clean the text from the catalog side and completely clear out the admin in source view and then re-add the cleaned code.
~Melanie