Joss Whedon claims he didn't put a lot of Easter eggs in Avengers: Age of Ultron, but he couldn't resist at least one.
According to Screencrush, the reference in question is to Whedon's best-known property (before writing and directing the two Avengers films), Buffy the Vampire Slayer. And it comes during a scene that is already so difficult to comprehend that no one could blame you for missing it.
The scene is that much-discussed "Thor takes a swim" sequence in which the God of Thunder (Chris Hemsworth) dips into a magic pool that lets him watch a trailer for Thor: Ragnanok see a vision of the doom that's coming to Asgard. As the Muscled One is assaulted with images of all kinds of ruin and malevolence, there's a quick -- and I mean quick -- glimpse of three masked beings in an archway. Luckily for us, Screencrush grabbed the shot for closer inspection:
Turns out those three fellas are wearing, from left to right, a wolf mask, a ram mask and a stag mask. But since the medieval term for stag is "hart," that results in Wolf, Ram and Hart.
On the Buffy spin-off Angel, the Wolf, the Ram and the Hart were three ancient demons who operate in the present day as Wolfram & Hart, a law firm that specializes in representing killers and other evildoers.
And if you think this is unintentional or a stretch, Whedon himself confirms it on the director's commentary for Avengers: Age of Ultron, which comes out on Blu-ray and DVD on October 2 (it's already available on digital HD).
So Buffy/Angel fans, take heart: Joss knows you're still out there and couldn't pass up the opportunity to plant a little in-joke from his second-biggest franchise into his first. Now let's start hunting for a Firefly reference...