There’s an on-screen Tweet at the beginning of Missing (2023) that foreshadows a major plot twist (explanation in comments)

In Freddy vs. Jason (2003) the name of the news station on the TV at Westin Hills is a nod to Freddy Krueger's last name.

In The Burbs (1989), there is a box of Gremlins cereal on the counter. Both The Burbs and Gremlins were directed by Joe Dante.

In The Lord of the Rings: The Fellowship of The Ring (2001), after the hobbits fall down a hill, Merry says "That was just a detour, a shortcut." Sam asks "A shortcut to what?" and Pippin says "Mushrooms!" In the original book, chapter four is called "A Short Cut to Mushrooms".

In Rocky III (1982) one of the ALHS Drummers has a drum signed by Mr. T

In Lord of The Rings: The Two Towers (2002), one of the Elves at Helm's Deep is played by Daniel Falconer. He works for WETA workshop and actually designed the Elvish armour for the movie.

Aftersun (2022) — the opening shot showing miniDV footage of Calum features a faint reflection of Sophie, reminiscing on the videos she recorded of her childhood vacation.

The background animations in the opening credits of Scott Pilgrim vs the World (201 are related to the characters of the actors, and for the evil exes their number appears in the last frame.

In Patriots’ Day (2016), Dun Meng who fled a carjacking by the Tsarnaev brothers is played by Jimmy O’ Yang. In this scene the real Dun Meng is seen to the left.

In A Goofy Movie (1995), you can see in the diner scene that Goofy’s keychain is the Disney logo

In "The Peanuts Movie" (2015), the school paper's article about the talent show that Charlie Brown helped Sally win includes a quote from Sally claiming that she's going to Disneyland