Oblivious Janitor Cut

A recurring gag in action or speculative fiction works where something absolutely incredible is happening outside, but then we cut to the inside of an office building and we see that the janitor is completely oblivious to what is going on around him, either because he has his headphones on, because his back is turned or because the cleaning equipment he is operating is too loud.

A Twisted Sister Trope to Funny Background Event.

Not to be confused with The Almighty Janitor, who sees and knows all.

Examples of Oblivious Janitor Cut include:

Film

True Lies: The Janitor doesn't notice the jet flying around outside until it crashes into the window.
Devil: A man falls off the top of a building while the janitor's back is turned to the window.
Ghost Rider: The eponymous biker rides down the side of a office building, shattering glass windows all the way down, and the janitor, wearing headphones again, doesn't notice the broken glass until after Rider is long gone.
The Day After Tomorrow: A tornado suddenly and severely damages a building. The janitor doesn't notice until the lights go out. Justified in that he is in a windowless hallway.
Grosse Pointe Blank: The Ultimart clerk plays an arcade game while a gunfight rages all around him. He doesn't notice until the hero yanks him out of the shop before it explodes.
Super 8: Breen misses the Sheriff being taken by the monster just behind him while listening to his Walkman.
Stan Lee said in an interview that his cameo in The Amazing Spider Man reboot would be as a librarian who is oblivious to a fight between Spidey and the Lizard in the background.
Harry Potter and the Deathly Hallows Part 1: While the hero trio is attacked by Snatchers at the diner, the waiter/cook is in the kitchen, blithely listening to music on her headphones.
Gets egregious in X-Men 2, when a janitor walks past Mystique disguised as him.
There is one buffing the floors in I Am Number Four, but he dies with the floor buffer polishing the school floor with his blood.

Live Action Television

Coupling: In the last episode, Jane is so wrapped up in cleaning her flat (and setting the mood) for sex that she doesn't realize Oliver is trying oh so desperately to get in. It's only revealed toward the end of this scene that she's wearing headphones, which is why she hadn't noticed his clawing at the door, hanging outside her window, or trying to grab her from the mail slot.
Pushing Daisies: Olive breaks into a musical number while closing up the pie shop. Toward the end, the janitor pulls off his headphones, interrupting her song, and asks if she said something.
Chuck: In the episode "Chuck vs. the Push Mix", the camera shows a janitor buffing the floor in the foreground, and the long-awaited proposal from Chuck to Sarah in the background.
Doctor Who: The TV Movie: The Doctor and Grace pull off a dramatic escape from the Institute while being chased by the Master and his minion. The janitor fails to notice.

Video Games

In episode 3 of the Telltale game of Back to The Future Marty crashes the DeLorean into a billboard in A-1986. A young woman picking up trash on the highway (who later turns out to be none other than Marty's girlfriend Jennifer) has her back turned and her headphones on.

Web Original

The janitor in The Mercury Men is oblivious to the alien invasion going on in the building - that is, until he gets zapped by them.

Western Animation

The Spectacular Spider-Man: In the episode "Nature Vs. Nuture", Spidey stops a helicopter from crashing into an office building, and the janitor doesn't even turn around.
This is a running gag in the Robot Chicken Star Wars specials.
In the SpongeBob SquarePants episode, "Doing Time," there is a janitor listening to a headphones who is completely unfazed by the fact that a giant wave of cranberry juice just blasted through the hallway he was mopping.