I think this might be a bug.
 
When I click on the "Update Bug" button to update an existing bug, both the "Assigned To" drop-down and the "Product Version" drop-down are reverted to the first value of the drop-down.  They failed to retain the existing values.
 
I discovered this because I was going through and updating existing bug records.  Later found out that all the "Assigned" bugs were no longer assigned to anyone.  All the product versions were screwed up as well.  :-(
 
It would be best if "Update Bug" will retain the existing drop-down values.