When you fail to process animal skins in time, they will be rotten, unusable. Rotten [tanning stage] [animal name] skin.

When it's rotten, it's of no use. Burning them in fireplace doesn't work at all and Shift+D should be used instead to discard the item and delete it from the world. The fireplace however is a good use of other "rotten" items, for example you can use a inferior boards to feed the fire.