You should return borrowed items.
Sam forgot his calculator at home.
Sam wants to find a calculator he can use on his math test.
Sam borrows a calculator from his friends and gives it back once the class is over.
Sam borrows a calculator from his friend but doesn't return it after the test.
Sam and his friend are both able to use the calculator on their tests.
Sam's friend does not have their calculator when it's their turn to take the math test.