In Spider-Man: Homecoming, Michelle (MJ) is reading a book titled "Of Human Bondage", a story of an orphan boy sent to live with his aunt and uncle.
2. In Avengers: Infinity War, Mantis sets her hands like an actual praying mantis when sneaking on Thanos.
3. In Captain Marvel (2019), when Monica is changing the color of Carol's suit she first chooses a combination of red and gold, the same colors of DC's Shazam, who was previously known as Captain Marvel.
4. Black Widow casually saves both Falcon's and Captain America's lives in Captain America: The Winter Soldier. When Bucky is shooting at these 3, he targets black widow first. She dives forward. She then pushes cap and Sam's heads out of the line of fire (watch the head cushions explode).
5. After stabbing Thanos, Gamora's sword doesn't have any blood stain or blood drops on it indicating its a fake reality.
In Thor: Ragnarok, the carcass that Valkyrie lands on is a bilgesnipe, a creature described by Thor in The Avengers.
7. In The Avengers (2012), Pepper Potts is barefoot in stark tower whilst Tony Stark wears shoes. This was done to mitigate the height difference between Gwyneth Paltrow and Robert Downey Jr.
8. In Black Panther, Shuri asks which side of the road should she drive on due to her education in England as England drive left and most of the countries drive right.
9. Thor Ragnarök: Hela's bruises disappear once she reaches Asgard, along with the cracks on her skin. Previously it is mentioned that her power increases when she's on Asgard.
10. In Spider-Man: Homecoming, Aunt May is reading ‘The Woman in the Dunes’ by Kobo Abe - which is about an entomologist who gets caught like an insect - at a time in the plot where Peter Parker is almost ‘caught’ as Spider-Man.
In Ant-Man and The Wasp (2018) Ghost makes the motion of opening the double doors as she ghosts through them.
12. In Thor (2011), Queen Frigga is visibly uncomfortable when Odin states that Thor is his firstborn child. In Thor: Ragnarok (2017) we learn that Odin’s and Frigga’s firstborn was Hela.
13. When first introduced to Kate (a.k.a. Agent 13) in Captain America: The Winter Soldier she hangs up the phone and says to Steve she was on the phone to her aunt, who is kind of an insomniac. It is then revealed in Civil War that Kate is the niece of Peggy Carter.
14. During the casino brawl scene in "Black Panther," Nakia uses her high heels as a weapon. Later on, during the car chase scene, she is shown to be driving without her shoes
15. In Guardians of the Galaxy Vol. 2 (2017) Mantis states that Ego found her orphaned on her home planet. When the Ego is talking about The Expansion Ego can be seen with someone from the same race as Mantis as he was likely visiting trying to sire a child.
In Avengers Infinity War, Gamora reaches for her sword when Thor approaches her seemingly hostilely.
17. In Thor: Ragnarok, Hela reanimates dead Einherjar and Loki revives Surtur with the same spell.
18. In Age of Ultron, Ultron says Wanda can tear the Avengers apart from the inside. In Civil War, Wanda’s actions lead to the Accords and the split between Tony and Steve.
19. In Iron Man 2 (2010), Natasha Romanova (Black Widow) potentially blows her cover by adopting a defensive stance when Tony and Rhodey crash through the ceiling during their fight.
20. Iron-Man 2 (2010) - Black Widow's alias Natalie Rushman's resume includes experience at Hammer Industries, which is perhaps why (later in the film) she is already familiar with the security, building layout and computer systems at Hammer Industries.
In Captain Marvel (2019), Vers picks up the movie The Right Stuff (1983) inside the Blockbuster Video. The Right Stuff takes place at Edwards Air Force Base, which is where she was stationed while in the Air Force.
22. In Spider-Man: Homecoming, at the start of the scene where ‘MJ’ is revealed to the audience, a kid dressed as a tiger is seen running right through the middle of the frame. For fans of the comics, this is a lovely little piece of foreshadowing because MJ’s nickname for Pete is ‘Tiger’.
23. In Captain America: Civil War, part of Sharon Carter's speech from Peggy's, "Plant yourself like a tree" bit was taken directly from The Amazing Spiderman #537 when Captain America tells the same thing to Spider-Man.