Trivia: In frame by frames views of the scene where Vader lifts the Emperor and throws him down the shaft you can see x-rays of Darth Vader.
Trivia: When Mort smashes the shower door, the hole left in the glass looks like a small, four-legged animal with a screwdriver in its neck - which is how Shooter kills his dog.
Trivia: When Frank comes out of the newspaper building, he walks into a guy with piercings and t-shirt with the Grim Reaper on it, this is the Director, Peter Jackson, he does a Hitchcock in all his films. (00:25:30)
Trivia: When Burnett and Lowrey take a car to chase the van, the driver is American footballer Dan Marino. The driver of the red and white car who stops and calls Burnett a freak is the film's director Michael Bay. (01:22:35)
Trivia: Near the start of the film, when the first kid has been captured by Leatherface and they're in the basement, the decapitated head on a table beside Leatherface is that of Harry Knowles, film critic and gossip monger of Ain't It Cool News. (00:37:05)
Trivia: The bloated woman who is brought into the mall in the truck is actually played by a man. His name is Ermes Blarasin, and he is a stuntman.
Trivia: After the truck with the gold in it, drops from the street and people scatter and scream, there is a man dressed as Spider-Man on the street. (01:25:35)
Trivia: Bonnie Bedelia makes a cameo as John's ex-wife, Holly, via a picture of her driver's license. To illustrate the divorce, Holly even uses her maiden name, Gennero. (00:36:55)
Trivia: Once again, Stan Lee makes his usual cameo. This time, he appears as a judge at the Christmas pageant. (01:03:50)
Trivia: Kevin Peter Hall, who plays the Predator, also appears as the pilot of the general's helicopter at the end of the film.
Trivia: Stan Lee, the creator of Spider-Man, makes a cameo appearance when Peter Parker is reading about Spider-Man getting the key to the city. He is the man that walks up and talks to him.
Trivia: The Sator Square dates back to Pompeii and is a square of 5 Latin words making a palindrome: Sator, Arepo, Tenet, Opera, and Rotas. Whichever orientation the square is in the words line up in both directions. Each word features in the film - Kenneth Branagh's character is named Sator, the forger mentioned near the start is Arepo, the title is of course Tenet, the opening sequence takes place in an Opera, and the Ukranian special forces who arrive in that sequence have badges on their arms with "Rotas" on them.
Trivia: Near the end of the film, as Dani is lifted and being carried to the dinner table, if you look closely, you can see the ghostly image of her deceased sister's face (with the exhaust hose in her mouth) in the background. It's been superimposed over the trees in the distance. It's very subtle and easy to miss, but if you know where to look, it's there.
Trivia: Just after the first building in LA blows up, there is a close-up of a guy putting cash in a case and he's blown backwards in the blast. This is Volker Engel, the film's visual effects supervisor, who won the Oscar for Best Visual Effects for this film.