How does Zaroff injure his shoulder in "The Most Dangerous Game"?
After General Zaroff's announcement that his new favorite prey was of the human kind, Rainsford demanded to be released from the island. Instead, Zaroff decided that Rainsford would be his next prey. Rainsford was given an afternoon's head start, and he managed to survive the first night by sleeping in a tree. But when Rainsford realized that "The Cossack was the cat; he was the mouse," and that Zaroff had intentionally allowed him to escape in order to prolong the hunt, Rainsford knew he must attempt to kill Zaroff himself. Rainsford set a trap that he had learned in Asia--the Malay mancatcher. When Zaroff approached,
... the dead tree, delicately adjusted to rest on the cut living one, crashed down and struck the general a glancing blow on the shoulder as it fell... He staggered, but he did not fall... He stood there, rubbing his injured shoulder, and Rainsford, with fear again gripping his heart, heard the general's mocking laugh ring through the jungle.
Rainsford's trap did not kill the Russian, but it did buy him more time, since Zaroff had to return to his house to tend to his wound. Rainsford had more tricks up his sleeve when Zaroff next returned.