A clever trick!

Late summer wouldn't be complete without a spider photograph. This Garden Cross Spider Araneus diadematus lives in our garden. As you can see it has two unusually short legs on its right side. 
Should a spider be unfortunate enough to lose a leg, then provided it still has at least one more moult left in its life cycle it is able to grow a new leg over a period of weeks. This is probably what is happening to our spider.
Regeneration in spiders is not just limited to the legs. They also have the ability to grow new pedipalps, mouthparts and silk spinnerets.