I recovered a fortress, and there was a bunch of food in it. Now that food lies in a giant piles that my dwarves can't eat. They aren't marked forbidden or for dumping, they are just sitting there. I've tried marking them forbidden and dumping them in order to clear the status, but nothing. Just sitting there, delicious but inedible.
This is something that can happen to items that were required for a job (for food, that job was most likely hauling to a stockpile or barrel) at the point where the fortress fell or was abandoned. The items remain flagged as required for a job, but the job no longer exists, so it can't be completed or cancelled in order to clear the flag.
I seem to remember from discussions of this issue on the Bay12 forums that the flag could be cleared by mining the floor out from under the item and causing it to fall to the level below.