Biff's cane
From Futurepedia, the Back to the Future Wiki
Biff Tannen's cane was an object that he carried around with himself at old age in 2015. A metal fist was located on the top of cane, with which Biff would knock people on the head with, particularly Martin McFly Jr. , much like he did to his grandfather and father before him.
When Old Biff stole the DeLorean time machine, he brought his cane, and left it behind as he returned to the future. At this point, Biff was beginning to vanish from existence due to the new history he had created. As he exited the DeLorean, the top of his cane broke off and fell inside the car.
When Marty McFly and Emmett Brown left 2015 for 1985, the top of the cane was unknown to both, and they found themselves trapped in an alternate reality. Only after finding Biff's cane, along with a receipt to Blast from the Past, did they realize what had happened.
