In Person of Interest Season 1 episode 20, "Matsya Nyaya", John went undercover as a rookie under Tommy Clay. This episode is mostly themed about robbery and teammates killing each other. Like Tommy first kills his partner and shoots John in the back. Finally, Clay (Tommy's girlfriend and associate) kills him in the end to get the platinum packet.
Similarly, in another flasback story, John is given instructions to kill his partner after the job is done. He hesistes and his partner also got the same instructions. And the CIA does an air strike to kill them both to cover the tracks.
In a similar manner related to both, Captian Arthur got killed by his associate Fusco in rescuing John.
In the same episode, Tommy plays a prank on John by pretending the robbery is going on and he comes from a truck with a Joker mask. This mask is exactly as used by the robbers in 'The Dark Knight' in the intial sequence, where everyone has order to kill his associate.
Is it a co-incidence or is Jonathan Nolan giving tribute to his brother Christoper Nolan's movie The Dark Knight or maybe a foreshadowing?