In the database table CC_inventory, the 'updated' column holds a Date/Time data type. Under certain circumstances, this column holds 0000-00-00 00:00:00. (I don't know how it got that way.)
Queried by products.index.inc.php near line 1216, where later $updated_time is set to the column value, then used as a looks-like boolean.
Since the string contains characters other than zero/decimal, this results in a true result and the Product Inventory list displays that string. The expected display is a phrase that says: Unknown.
Suggest: ... = ($updated_time !== "0000-00-00 00:00:00") ? ...
(Just mentioning it here until there is a publicly accessible bug tracker put online.)