Whenever a movie has anything that remotely looks like military tech, the producers turn to the actual military for help with gear that'll look the part. But when Marvel came a-calling with The Avengers, the Pentagon decided not to play ball with Joss Whedon and company. And their reasoning is priceless.
The DoD was totally on board with The Avengers until midway through production, when, according to U.S. Defense Department Hollywood liaison Phil Strub, it became confusing to the powers that be where, exactly, the Strategic Homeland Intervention, Enforcement and Logistics Division would fall in the current bureaucratic command structure:
"We couldn't reconcile the unreality of this international organization and our place in it. To whom did S.H.I.E.L.D. answer? Did we work for S.H.I.E.L.D.? We hit that roadblock and decided we couldn't do anything [with Avengers]."
According to Wired, Marvel tried to explain—with what must've been a straight face—that given the word "Homeland" in its title, S.H.I.E.L.D. would be part of the Department of Homeland Security, not the Department of Defense. If S.H.I.E.L.D. was a real organization. Which it is not. Because The Avengers is a work of fiction.