During Anakin's transformation into Darth Vader in Revenge of the Sith, Anakin's heartbeat can be heard in the background until Vader's helmet is put on. Anakin dies the moment Vader is born.
In Avengers: Infinity War, Thanos has to use all four of his infinity stones to defeat Dr. Strange, sometimes using two stones at a time to get past Strange's spells.
In Thor: Ragnarok Hela’s headdress gets larger the longer she's on Asgard. The rips on her costume disappear and the bruises on her body change color from dull silver to shining green. Previously it is mentioned that her power increases when she's on Asgard.
In Spider-Man 3 (2007) you can see Sandman's teeth fly out when his head is pressed against the moving subway train by Spider-Man.
In Ant-Man, Darren Cross's tie has a hexagonal pattern foreshadowing his real identity as Yellowjacket (the villain of the movie and a type of wasps which live in nests that have hexagonal cells).
In "The Dark Knight" you can see the piece of glass the Joker uses to escape his cell, while he is talking to Detective Stephens.
In Iron Man (2008) when Pepper Potts is going through Obadiah Stane’s computer, played by Jeff Bridges, there are two references to Bridge’s Big Lebowski character. In the documents, there is a reference of a vessel called ‘MSC LEBOWSKI’ and a barcode that includes ‘THE00DUDE’.
In Black Panther, after Killmonger takes the throne, he begins to wear his father's ring on his right hand rather than on the necklace.
In Monty Python and The Holy Grail, the Black Knight’s coat of arms is a boar, a creature known for ignoring deadly wounds to keep fighting.
In It (2017), Pennywise changes the color of his eyes from yellow to blue, which are the same color as Bill's, to lure Georgie. Also, during the sewer drain scene, Pennywise has one eye staring at Georgie while the other eye stares at the audience.