Metal Detector Checkpoint

I pity the fool going through airport security with 50 pounds of chains!

In high security areas, such as airports, people will have to go through a metal detector before continuing on. A few certain things can happen:

The person going through the detector or security personnel end up doing something silly as a result of inattentiveness. For instance, a person sets off a detector, repeatedly tries to get rid of all their metal possessions, is repeatedly searched, and a comically long time passes until they remember some totally innocent and inconspicuous metal object somewhere on their person, or a metal plate in their head, or some such.
Rather than being a silly mistake, it's an illustration of there being something weird about the character, as they turn out not to be carrying any metal objects despite setting off the detector, and security personnel and possibly the person themselves become increasingly confused as metal is detected where there is absolutely no good reason for there to be any, hinting that something weird is going on.
The person has a metal implant in their body.
Another variety is when two people walk through a metal detector in quick succession, and the obviously wrong one gets hassled by security. Say, a cartoonish shady character in sunglasses with lots of suspicious weapon-shaped lumps visible under their long-coat and a ticking suit-case walks through, followed closely by a kind-looking little old lady with grey hair. The security personnel ignore the former, and throw the latter to the ground in a panic, possibly while screaming for backup. Hilarity Ensues.

Examples of Metal Detector Checkpoint include:

Advertising

An ad for underwear had two female security guards repeatedly setting off the metal detector with a remote in order to get a handsome man to strip down to his underpants.
One commercial for Crash Bandicoot 2 as Crash (Actually a man in a Crash costume) going past an airpoit metal detector, and repeatedly setting it off. At the end, he turns his back and we see he's wearing a jet pack.

Anime and Manga

At one point in Death Note, part of the Kira Investigation Squad ends up having to take their pants off to get through a metal detector.
In one episode of Golgo 13, Golgo has to go through a metal detector with his gun. He gets a toy pistol made entirely out of plastic for the job, (It would only be good for one shot, but then again Golgo only needs one shot) and walked through with the bullet hidden in his mouth, where any investigator would assume the detector is sensing some dental work.

Comics

In an issue of The Defenders, when Dr. Strange passes through a metal detector in his secret ID the Eye of Agamotto sets it off. He habitually kept it invisible but forgot that his spell didn't work on machines. He used a quick illusion spell to make it look like a St. Christopher medal.
In issue 22 v.1 of his first series, Cable goes through metal detectors at the airport and gets stopped by security, only for him to explain it's his keys. This coming from a man known to pack heat and has a metal arm. Domino points this out and Cable replies that he'd never "bring a gun through airport security. At least not a metal one..."
- Later in the same issue, Shatterstar tries to take his swords through airport security, but the security guard confiscates them until he leaves. The TSA was strict even back then.

Fan Works

In the Iron Man fic Check Point, Tony and Pepper want to fly on a public airplane so that Tony can get closer to the common people. Unfortunately, Tony forgot that his arc reactor can set off metal detectors.
This fanart: what happens when a Naruto jounin goes through security? Hammerspace has its disadvantages.
Luth has fan art with Ferrus Manus going through metal detector at an airport. Right in Power Armour, not that removing it would help.

Film

Die Hard: Somehow, a Glock 7 doesn't show up on your airport metal detectors.
- Justified in the first one, I don't think we see him carry the gun into the airport, we just see him on the plane with one, since he is a police officer. Under certain circumstances (possibly under even more circumstances back in the 80s) on and off duty police officers may carry weapons on a plane, though usually they have to leave them with the pilot.
The 70's dystopian film Class Of 86 has these outside of a school as visual short hand of how bad society has fallen.
Neo and Trinity go through one in the first Matrix movie. Security guard asks Neo to open his coat... and More Dakka is unleashed.
Moonraker: when Jaws goes through an airport metal detector his metal teeth set it off. When a guard tries to stop him, Jaws just grins at him and the terrified guard lets him pass.
In Penn and Teller Get Killed, Penn & Teller play tricks on each other throughout the film. At an airport Teller does stuff to make Penn look guilty at the metal detector. Link.
In A Fish Called Wanda Kevin Kline's character gets a gun past an airport metal detector by tossing it around the outside as he passes through. Riiiiight...
In Wallace and Gromit: A Matter Of Loaf And Death Gromit installs one of these (based on a design in "Electronic Surveillance for Dogs") to catch out Piella's murderous intentions toward Wallace. The machine works but fails to convince Wallace.
The Golden Child: Chandler Jarrell needs to get the Ajanti Dagger through an airport metal detector in Tibet. He slips the Dagger into the pocket of another passenger, then the unwitting dupe goes through and the alarm sounds. Chandler pretends to be a law enforcement official, "confiscates" the Dagger on the other side of the detector and continues on his way.
Parodied (of course) in the second Airplane! movie, when a bunch of terrorist-y thugs stroll through the airport metal detector carrying huge weapons without setting off the detector, but then an elderly woman sets it off and gets thrown up against the wall and searched.
In the movie In the Line of Fire, the assassin has to pass a metal detector to reach the president.
In The Fugitive, the Marshalls bypass the metal detectors in the Cook County Jail building by flashing their badges.
This Is Spinal Tap: the scene with the zucchini in tinfoil.
Happens twice in Léon a.k.a. Léon: The Professional:
- When Mathilda needs to get past the metal detector at the DEA building, she hides a weapon in the take out food bags she is delivering.
- When Leon goes to rescue Mathilda from the DEA building he passes through the metal detector. The arsenal he's carrying sets it off, but he punches out the guard before the guard can do anything about it.
In X-Men, the characters pass through one of these entering a museum. Wolverine, naturally, sets it off, and then destroys it.
In Police Academy 5, the team is traveling via commercial airline-including the insufferable Capt. Harris and his lackey, Proctor. Ventriloquist trickster Larvelle Jones (Michael Winslow) throws his voice to imitate a metal detector alarm every time Harris passes through it. Harris obligingly surrenders metal from his body, including keys, wallet, watch, swagger stick, belt buckle (this last with a bit of overenthusiastic help from Proctor), (although conspicuously forgetting badge, coat buttons, and officer's hat-did I mention he was traveling in uniform...well, Rule of Funny) but the alarm continues to "go off". Then a distraction causes him to forget to use his hands to hold up his now beltless pants...
In She's Out of My League, a perverted airport security guard sets of the metal detector when pretty women pass through it to force them to undress.

Live Action TV

The X-Files: Agent Scully going through airport security and triggers the metal detector. This leads to the discovery of the computer chip implanted in her during her abduction.
In The Sarah Connor Chronicles when John & Cameron go to their new school after traveling from 1997 to 2007, they encounter in-school metal detectors for the first time. Cameron (being a Terminator) naturally sets off the detector. John says that she's his sister and she has a metal plate in her head from a childhood injury, a story helped by Cameron's Uncanny Valley nature.
In Leverage, a corrupt lawyer is delayed by stuffing him full of keys in all of his various pockets before he can enter the court house security check.
In the Airwolf episode "Flight #093 is Missing", Caitlin is going through security at an airport and has to take a number of wedding presents (for her sister's wedding) out of her bag. "She's from Texas" is Stringfellow's response.
Shows up in Firefly; it's linked to a force field, so a socialite is stopped until he surrenders his derringer.

Tabletop RPG

In Shadowrun metal detectors are a security device that runners must be wary of. They're a standard feature in secured buildings and can detect the weapons and other metal devices runners often use.
The walkthrough metal detector from GURPS: High-Tech gives a variable bonus to the operator's Search skill. The description notes that security teams don't usually have the sensitivity turned all the way up because it can end up catching even tiny bits of metal and lead to long lines.

Video Games

Left 4 Dead: the Dead Air level has these. Given the human characters are carrying guns, you best avoid these. If you don't, be prepared for a swarm a zombies to come rushing at you and your friends due to being attracted by the high pitched alarm.
In Second Sight, you are able to bypass one (since your Invisibility power is actually a Charm Person ability it isn't much use against machines) by using telekinesis to manipulate a loose connection (although anyone who didn't hear a conversation between two guards would think using telekinesis on it just stops it working for some reason).
In Hitman: Blood Money, your final mission requires you to infiltrate the White House as part of a tour-group. The main challenge is the metal-detector/X-ray checkpoint. There's a number of ways to smuggle weapons inside - your Sniper Rifle can be upgraded with a special foil-covered Attache Case which can glide through the X-Ray, but you can also smuggle in your trusty Silverballers - by slipping them into the luggage of a middle-aged woman. When she gets nabbed at the checkpoint, the guns are confiscated, and you can then retrieve them from the lock-up later (after you've 'borrowed' a Marine Uniform, preferably.)
- Hotel in Hitman: Contracts has a few, including the door. So much for being given a cool sniper rifle. Though it's possible to smuggle it in, and it's possible to smuggle the bomb out (it's a mission objective). A security uniform helps (they are supposed to be armed, of course).
  - Thanks to Artificial Stupidity, the easiest way to get things through this metal detector is to put them down, walk through without setting it off... and then pick them up again from across the rope barrier.
- In Hitman 2 you have to dress up as a fireman and launch a fire alarm to enter through one of these; those fireman's axes have a lot of secondary uses...
You have to avoid these in the airport from Splinter Cell: Pandora Tomorrow's final level, because Sam Fisher carries lots of metal gadgets and weapons. Naturally, they set off an alarm if you pass through them.
In Police Quest II, just make sure to show your police ID when going through the airport metal detector.
The Intensive Treatment Unit in Batman: Arkham Asylum uses some sort of high-tech x-ray chamber, but the Arkham Mansion has a Metal Detector Checkpoint on one side of a corridor intersection. Aside from alerting some gun-toting Mooks that you're here, it serves no purpose at all.
Resident Evil Code: Veronica has one of these that requires you to put your items in a box that isn't linked to the normal item boxes. Don't forget that empty fire extinguisher.

Web Comics

Vexxarr has fun with a standard airport security guy checking an armored robot for random metal objects.
Monica of Wapsi Square has a bit of a problem with the metal detector at the airport. She made sure she had nothing metal on her person only to set off the detector with the underwire in her bra.
Lothar in Exterminatus Now triggers these... what they expected from a cyborg with three appendages and part of the head quite visibly not his own?
Sinfest has Thought Detector Checkpoint. In hell.

Western Animation

The Simpsons: in the episode in which Homer got a handgun, he took Marge to an NRA meeting. Corney tried to go through without a gun, was turned back, handed a gun, and made to go through again to ensure that the detector beeped.
In Rocko's Modern Life, Rocko is stripped down to his underwear after setting off the metal detector. This after several characters with metal in their persons (including a chainsaw-wielding maniac and a Tin Man) go through without the machine going off.
This Cartoon Network promo for Secret Squirrel.
Phineas and Ferb: implied when Major Monogram mentions having a lot of metals in him by Dr Doofenshmirtz. It is unclear was Monogram Metaphorgotten or serious.