Remove Category from Item

I'm trying to remove a category from an item that has two listed. When ever I edit the item, deselect one category, save... it still shows up. When I got back to edit the item again, both categories are selected as before.

Thanks for any help you can provide for this.

hello
did you desactived plugin for mootools compatibility ?

Maybe the primary category is set to which you want to change.

Thanks