In Rain Man (1988), Raymond memorized names from a phone book, the last name is "William Gottsegen". This is actually the name of Dustin Hoffman's real life father in-law.

In The Triplets of Belleville (2003), when Madame Souza arrives at the triplets' flat, the turd floating in the toilet bowl is shaped like Mickey Mouse

The two kids who hand Spider-Man his mask back on the train in Spider-Man 2(2004) are Tobey Maguire's real life half-brothers

In Shin Godzilla (2016), there is a very brief moment during a Twitter sequence where you can spot a profile called @bakashinji with an Asuka Langley profile pic. The director of this film, Hideaki Anno, also directed Neon Genesis Evangelion (1995)

In A Simple Favor (2018), Blake Lively drinks Aviation gin, a company her husband, Ryan Reynolds, owns a portion of.

In Eyes Wide Shut (1999), Dr. Bill picks up a newspaper with the headline “Lucky To Be Alive” (explanation in comments)

That Thing You Do (1996)

What about it?

When The Oneders play at The Orpheum theater one of the other acts on the marquee is Marilyn Lovell and The Geminis, a reference to Tom Hanks earlier film, Apollo 13

In Scream VI (2023), Tara has a scar on the back of her hand from where the knife went through in the opening scene of Scream (2022)

In the Lego Batman movie(2017)Harley Quinn’s shirt says “smylex”the product used by joker in Batman 1989

What about it?

Image of official minifigure

TIL that The Incredibles (2004) is set in 1962

Dungeons & Dragons: Honor Among Thieves (2023) 'Son of a Continuity' (Spoilers! Detail in the comment)