Betrayal Insurance

Batman: What would stop you from doing what the Lorder Superman did?
Superman: There's always that kryptonite you carry around.

—Justice League Unlimited

Your friend reveals that he knows of some weakness that you have, or has a tactic or piece of Applied Phlebotinum in reserve specifically to take you down; you know, "just in case".

Maybe he has been betrayed before, maybe he doesn't trust anyone at all, or maybe he simply thinks you are too dangerous, might someday give in to the temptation to abuse your powers, or most benevolently, Mind Control is real in this world, and you don't have any immunity to it. Whatever his motives, he's sitting on Hidden Supplies of the necessary items or substances to put you down in an instant if the need arises.

Regardless of why the Betrayal Insurance policy was originally created, it is rare for it to be used.

How you feel when you find out your friend has a policy out on you will vary. Mostly depending on where the show you're in falls on the Sliding Scale of Idealism Versus Cynicism.

This also occurs when the betrayal has already happened and one of the betrayed reveals he has something for just this situation.

Compare to Kryptonite Ring, where someone personally takes precautions in case he ever goes bad himself.

Examples of Betrayal Insurance include:

Anime and Manga

Late in Zettai Karen Children, Minamoto is ordered to always carry the gun with which he kills Kaoru in the precog's visions, notably he feels pretty conflicted about it.
In One Piece, Ivankov allows Crocodile to join in the break-out of Impel Down, because he knows a "terrible weakness" of his that he will expose if Crocodile backstabs them.

Comic Books

Professor Xavier also had a set of plans on how to stop the X-Men, Xavier himself being the subject of the first entry; however, these have rarely been mentioned since they were introduced.
In Runaways, Chase gives Niko a list of Logic Bomb questions that will shut their resident robot down if he ever does the Face Heel Turn that a friendly time traveller warned them about. Given Niko's oft-stated feelings about the possibility of any of her friends betraying the group again no one should be surprised if she has spells thought up for all of her comrades.
This specific dynamic, Batman holding Kryptonite, appears again and again, especially in the DCAU.
- In the JLA story-arc "Tower of Babel", it is revealed that Batman has very, very cynical methods set away in case any of the Justice League members go rogue. The plans are stolen and used against them by Ra's al Ghul. The enemy starts by using a sort of third-party Kryptonite Ring against Batman: stealing his parents' bodies from the cemetery. This works to distract the Bat, drives him nearly mad, and leads him to abandon the JLA while searching for the culprit. After it's all said and done, the JLA (who had no idea about the plans) are pissed with Batman and consider throwing him out of the League. His plans are the following:
  - For Superman, there's an altered version of Kryptonite that shortens its half-life and turns it red. Though less lethal to Superman than the green variety, it turns his skin clear, making his body absorb huge amounts of yellow sunlight. This overpowers him so it takes all his will to keep himself from destroying everything around him.
    - It should be noted that if Superman had gone EVIL, this would be a terrible, terrible plan as it relies on his desire to not harm people to incapacitate him. If he didn't care about hurting people, it would just be making him even more powerful.
  - Martian Manhunter gets his skin coated with nanites that convert loose atoms into magnesium. This magnesium spontaneously combusts on contact with air, trapping Manhunter in a ball of fire.
  - For Wonder Woman, Batman knows that her determination in battle will never die. A device makes her hallucinate so she thinks she's facing unending waves of enemies, with no chance ever to rest. She can't interact with the rest of the world, and eventually her heart will give out.
  - Green Lantern gets a mental suggestion that makes him block his eyesight with his power ring. Blinded, he can't use his ring for anything else.
  - For the Flash, there is an implant that gives him constant uncontrollable seizures — at light speed. After 20 minutes of seizures he thinks he's been in them for days, in such pain that he prayed just to die.
  - Aquaman is dosed with an altered version of the Scarecrow's fear toxin. He becomes terrified of water, leaving him dehydrated and due to die in a matter of hours.
  - Plastic Man is frozen solid and smashed to bits.
  - What's also interesting is that most of these use tech from Batman's own Rogues Gallery. Plastic Man's comes from Mr Freeze. Aquaman's comes from Scarecrow. Wonder Woman's comes from the Mad Hatter. Manhunter's may come from Killer Moth, and Green Lantern's may come from Hugo Strange. It's possible that Deathstroke's confiscated tech was used against the Flash.
- The Batman/Superman comic combines the this and Kryptonite Ring: After a long-term mission to destroy all the kryptonite on Earth, Superman saves one piece, which he gives to Batman for emergencies. Batman takes it to a lead-lined room in the deepest area of the Batcave... which is already filled with multiple kinds of kryptonite!
A very dark version occurs in Irredeemable, where Badass Normal the Hornet stops trusting his team's Superman-expy the Plutionian when the latter refers to the Hornet's wife by name, which the Hornet hadn't revealed. Realising that this ally with god-like powers is spying on his team-mates, and that the Plutionian has a human personality with human faults, the Hornet looks for weapons against him.
- His chance comes when Earth is invaded by an alien conquering race called the Vespa. The Hornet offers them teleportation technology (incredibly rare in this universe) and the co-ordinates of peaceful and defenceless planets his team has encountered, on the promise that the Vespa won't attack Earth, and will return to fight the Plutonian if the Hornet asks them to. The plan works, with the Plutonian captured and imprisoned by the Vespa. Temporariliy at least.
Green Lantern; it is not widely known, but a Power Ring chooses its wielder (not the other way around) and a Lantern does not have to actually wear the ring to control it. Anyone who tries to disarm a Lantern of their ring or - even worse - try to use a stolen ring learns that the hard way.

Film

In the Star Wars mythos, Force Lightning - a Dark Side technique used by Palpatine and Count Dooku - is one of the deadliest and most evil ways to channel the Force. Anyone who has seen Revenge of the Sith knows that the reason Palpatine is a bent, twisted, ugly old man is because his Force Lightning was reflected back upon him in the battle with Mace Windu. Palpatine made certain that his apprentice would not betray him by making sure Darth Vader's armor and cybernetics were especially vulnerable to this technique, one reason Vader never used it himself. And ultimately, Vader was indeed killed due to this weakness when he betrayed his master, just not in the way Palpatine expected.

Literature

The Anita Blake series has characters set such things up for themselves when they realize how close to the edge they're getting. Anita is none too happy about being Jean-Claude's, and it's implied later that it's because she has to find someone else to do this in case she ever goes bad.

Live-Action TV

On Smallville, Chloe has (or had) caches of Kryptonite stashed around the world, in case Clark goes bad again. Clark didn't know about them for a while.

Newspaper Comics

From Dilbert; when the robot gloats that it will, someday, take over Dilbert's job, Dilbert reveals the self-destruct program is has.

Western Animation

Much like the comics, animated Batman, in all different forms, practices this trope.
- In Batman Beyond, the aging Bruce Wayne seems to have been keeping a piece of Kryptonite locked away for years just in case. Possibly, this was the same piece he uses against A.M.A.Z.O. in Justice League when it copies Superman's powers (and weaknesses.)

Hawkgirl: Do you always keep that in your belt?
Batman: Call it...insurance. (grapples away)
Hawkgirl: And they call me scary.

The Batman has an incredibly cynical version. When Batman got a piece of kryptonite off of Metallo, not only did he keep it, he lied about giving it back (he gave Supes a fake one and kept the real one). He did it to "get even" with Superman for finding out his identity. However, by the end of the episode, it's the idealistic version, as they have settled their differences, and when Batman offers to give Supes the real kryptonite, Superman says to keep it. It still goes to show you that Batman really doesn't like anyone being nearly as Crazy Prepared as him.
This is also used by the end of The Batman: Batman gives each member weapons that simulate each Justice League member's weaknesses because aliens have invaded and copied their powers.
Batman makes an oblique reference to having Kryptonite in reserve in Justic League: The New Frontier, when he warns J'onn J'onzz that he [Batman] knows his weakness [fire], too.

Batman: I have a seventy-thousand dollar sliver of radioactive meteor to stop the one in Metropolis. With you, all I need is a penny for a book of matches.

In the Legion of Super-Heroes animated series, it's Brainiac 5, otherwise portrayed as Superman's very loyal Robot Buddy, who keeps Kryptonite for emergencies. This is very much the cynical version; Superman not only doesn't know Brainy has it, but he doesn't even know Kryptonite exists yet.
- Although Brainy seems conflicted about having the stuff; after it's been stolen, he says he's glad to be rid of it.
- What all of these oh-so-clever Kryptonite ploys always seem to forget is that Superman can completely protect himself from its effects with a very common substance: lead. I'd love to see a story where Batman pulls out the green rock only for Superman to wrap himself in his cape, say simply "lead-lined", and then deck him.
- More to the point, Superman has any number of ways of taking out his would be killers before they have a chance to pull out the Kryptonite. All the plans are dependent on Superman being a public menace prior to taking out the guy with the Kryptonite. Logically, if he went bad, killing Batman would be his first act.
- Superman tried using lead against Metallo in the Superboy live-action series. Metallo went all can-opener on him in a matter of seconds.
In Generator Rex, White Knight uses a mechasuit when he is forced to leave his sanctuary in Providence to help Rex with a world wide problem. Rex realizes that the suit was in fact originally designed to take him out if he ever went rogue on Providence.