The latest Magento release included a bug that was found today.
The bug will cause products to be deleted completely from the system when a frontend customer adds items to compare products and then clears all selections.
This bug was found in the latest Magento release, Version 1.0.19700, and is not known to affect version 1.0.
It is high priority for all people using Magento 1.0.19700, or who have upgraded to Magento 1.0.19700, to install this Patch.
Installing the patch:
1. Download either the zip file: patch1_1.0.19700.zip or the tar.gz file patch1_1.0.19700.tar.gz
2. Extract and upload the file CompareController.php or extract directly the archives to app/code/core/Mage/Catalog/controllers/Product/