There were a lot of them, and it was late, so don't expect us to have them all...but we think we did a pretty respectable job.
Check 'em out below and let us know if you caught anything we missed.
During the initial logo screen ahead of the movie, the 20th Century Fox fanfare fade, as the famed logo for the studio disappeared into darkness...all except the "X."
The Usual Suspects
This one isn't really an X-Men reference or anything, but the Bad Hat Harry logo has been altered between whenever I saw it last and now to resemble the linup scene from X-Men: Days of Future Past director Bryan Singer's The Usual Suspects.
Back to the beginning
The first X-Men movie featured a look at Magneto's childhood in a concentration camp, and he's once again back in a stark, numbered prison uniform when we first catch up with Michael Fassbender's version here.
There is, in fact, no scarcity of references to Singer's X-Men and X2: X-Men United, with the characters' time at the chess board just one such nod.
One of the handful of mutants who doesn't play a huge role, didn't get his own promotional featurette or movie poster, is Ink. Created by Arrow Executive Producer Marc Guggenheim, the character is one of those with Havok in Saigon.
Same deal; he's hanging out with Havok, Toad and Ink when Mystique comes to rescue them in Vietnam. This character (unless I'm misinterpreting who it's supposed to be) was made for the X-Men: Evolution animated series.
Quicksilver is apparently Magneto's kid. Who knew?
Well, it's implied anyway. Not really stated. At one point when rescuing Magneto from his cell, Quicksilver says, "You can control metal? My mom knew someone who could do that." In the comics, of course, Magneto is the father of Quicksilver and Scarlet Witch...and it's hard to guess why they wouldn't take advantage of that in the X-Men films since it's one of the few things that they can do and Marvel cannot in Avengers: Age of Ultron.
When Magneto goes to steal his helmet back, audiences can also get a glimpse of the coin that Magneto used to kill Sebastian Shaw, Havok’s damaged harness, and one of Angel Salvadore’s wings.
Through exposition, we also learn that Angel Salvadore, Banshee, Emma Frost, and Azazel were all killed at some point between the events of First Class and Days of Future Past.
Future tense nods
There are plenty of moments in this film that feel like callbacks to other dystopian future stories, including Fritz Lang's Metrpolis (there were echoes of this in the mechanized and brutal future of the Sentinels) and The Terminator (whose map for time travel was seemingly the inspiration for Singer, since it isn't really the Marvel Way, so to speak).
WhatCulture, who agrees with me on both of the above, also noted a third that I haven't seen anybody else spot, and would never have noticed: the scar pattern formed on Wolverine's chest when he first arrives in 1973 and is shot up, resembles a similar pattern on a character from Fist of the North Star. According to the story, "that anime is set in a post-nuclear apocalypse world, where a lone saviour rises to stop the oppression of others by a powerful minority."
There's even a brief clip from an episode of Star Trek in which Kirk talks about time travel.
As we reported earlier today, the list of cameos in the film is pretty significant. Besides the mutants with small but notable roles mentioned above, cameo appearances by characters included a number of actors from X-Men, X2: X-Men United and X-Men: The Last Stand as well as comic book creators Chris Claremont and Len Wein and filmmakers Bryan Singer and Newton Thomas Sigel.
Don't try to think too hard about his timeline, or you'll make yourself dizzy...but William Stryker has appeared in X2: X-Men United (played by Brian Cox) and X-Men Origins: Wolverine (played by Danny Huston). It seems as though even before the massive retcon at the end of Days of Future Past, they had elected to ignore some history, as the only flashbacks I remember seeing to Stryker in previous films were from X2.
At the end of the film, rather than Kitty Pryde and Iceman (who seemed teased as a couple throughout the Sentinel-dominated future framing sequences in the film), we see Iceman back with Rogue, as he was in the first couple of movies, and Kitty Pryde apparently dating Colossus, with whom she had a long and profound relationship in the comics.
There are more Xes in this movie than there were on the comic shops shelves in the '90s. Some examples I spotted: The light that allows entry into Cerebro, the fountain outside of the mansion, a reel-to-reel tape player in Xavier's office and Wolverine's belt in 1973. Where did you spot a hidden X?
As we previously noted, the crowd shouting for "En Sabah Nur" and the four men on horseback are a nod to Apocalypse, a powerful X-Men villain who will be the antagonist for the next film.