2013's Thor: The Dark World has a secret X-Men Easter egg. Last year, Disney completed their purchase of the bulk of Fox's film and TV empire. Marvel has regained the film rights to all the various X-Men and Fantastic Four characters, and they're eager to incorporate the X-Men into the MCU.
Oddly enough, Marvel Studios has managed to get in an occasional X-Men Easter egg. The most prominent was a reference to the SR-71 Blackbird Jet in the first Iron Man film. In the comics, the Blackbird has traditionally been associated with the X-Men since 1977's Uncanny X-Men #104. But this is one of the more obvious; there is, in fact, another Easter egg in Thor: The Dark World that explicitly references events tied to a powerful mutant.
One of the more amusing scenes in Thor: The Dark World saw physicist Erik Selvig explain the theories underlying the Convergence to a group of inmates at an asylum. It's most famous as the film's Stan Lee scene, with the famous comic book writer playing another inmate whose shoe has been purloined by Selvig in order to conduct his demonstration. The chalkboard is littered with references to the Marvel Multiverse - one of which is a phenomenon called the Fault.
In the pages of Marvel Comics, the Fault was created at the end of a devastating war between the Shi'ar and the Kree. The Shi'ar had been conquered by the powerful Omega mutant named Vulcan, and he had triggered a galactic war. In the final battle between the Shi'ar and the Kree, Vulcan engaged the Inhuman named Black Bolt in one-on-one battle. The Kree detonated a device called the T-Bomb, which tore a hole in the fabric of time and space. It created the Fault, an enduring tear in reality that opened the way to a twisted realm called the Cancerverse. It's still playing an important role in Jonathan Hickman's X-Men relaunch, having become a breeding ground for a monstrous alien race called the Brood.
Of course, it's impossible to know whether or not the MCU version of the Fault was created the same way. Still, it's an amusing - if indirect - reference to one of the most powerful Omega mutants, a villain who conquered an entire alien empire and set the galaxy aflame with the fires of war. It will be fascinating to see whether Thor: The Dark World's the Fault reference is remembered, perhaps as a key means of traveling between different realities.