Only happens after an item has been on a sale....
If you go into edit an item, the sale box is ticked with the last sale details you had on the item still ticked, so if you don't notice it, the item will then become a sale item again.
The only reason I noticed it was when adjusting a quantity of an item, it just caught my eye.
I've since done a test edit on other auctions and the same happens each time where the tick box is still ticked even though the item/s are no longer in a sale.