When young Agent K (Josh Brolin) first appears, he says "We'll take it from here", the first line K said in Men in Black (1997).
Young Agent K is wearing the watch that will become the light source for the Grand Central Station locker creatures in Men in Black II (2002). This can be seen clearly when young Agent K and J are having pie before the Mets fans enter.
The zip line escape system shown at the Apollo launch pad really did exist. It was installed for the Apollo program and enhanced for the Space Shuttle program. In some pre-launch emergency scenarios, the crew would have ridden steel cages down the zip lines to explosion-proof bunkers. Astronauts practiced using the system as part of their training, but it was never used in an actual emergency.
Upon first hearing Josh Brolin's impression of Tommy Lee Jones, director Barry Sonnenfeld burst into tears of relief.
Shea Stadium, which also appeared in Men in Black (1997), had to be re-created through CGI for this movie, as the stadium was demolished in 2008.
Chloe Sonnenfeld: Barry Sonnenfeld's daughter appears as the hippie girl who meets 1969 Boris at Coney Island. She also appeared as Elizabeth, the young Rugrats (1991)-loving girl at the post office in Men in Black II (2002).
Rick Baker: Special Make-up Effects artist, appeared as the alien with an exposed cranium at Z's funeral.
Rip Torn: As Large-Headed Alien at the funeral of Zed, Torn's own character from earlier installments.
Judy Murdock: Will Smith's personal make-up artist, appeared as a blue-skinned alien at Zed's funeral.
Barry Sonnenfeld: The couple watching the Apollo 11 launch is the director and his real-life wife Susan Ringo. Sonnenfeld is also one of the suicidal stock brokers in 1929.