Plot-Sensitive Items

In most fictional universes (and Real Life), things tend to function in a particular, expected way, and perform that function consistently. Phasers have settings, guns will go off when the trigger is pulled (and sometimes when it isn't), healing potions will heal, and cars will ignite as they fly through the air.

But sometimes the plot calls for an item to operate in ways that weren't previously established. This effect is unexpected, or even inconsistent with its previous known functions. But it'll get you through to the next plot point, and that's all that counts.

When a writer attempts to explain the sudden change of behavior, he/she will frequently invoke Phlebotinum Breakdown, or Retcon the function into the item's repertoire. The way to tell, frequently, is watching to see whether this new old function appears again.

If the change of function is not merely a handy means to an end, you may instead have Chekhov's Gun--particularly if the characters notice or should have noticed.

My Car Hates Me is a vehicle-specific subtrope, as Context Sensitive Button is for control panels. Another classic subtrope is Magic Countdown for time bombs and other critical events that goes faster or slower depending on how much tension is needed. The superhero version is New Powers as the Plot Demands. Not to be confused with Magic Tool and Green Rocks, both of which are known from the outset to have multiple and possibly unknown functions or the Green Lantern Ring which explicitly can do just about anything if you know how to use it. This is about a specific "normal" item acting strangely, not all-purpose phlebotinum like Star Trek's "reverse shield polarity" and "deflector pulse" or Doctor Who's sonic screwdriver.

Live Action TV

 * In the 1979 TV series Salvage 1, monohydrazine was used to solve any number of problems, from fueling a rocket to restarting oil wells.
 * Star Trek's automatic doors will always open (when not locked) when a person approaches, and shut as the person moves away--unless the door NEEDS to be wide open, in which case...
 * More to the point, the Main Deflector Array has been used to do just about everything.
 * It's even the go to choice when you want to do a one shot overloaded weapon attack. It does more damage than the actual weapons would.
 * The Stargate in the various Stargate series. Sometimes it closes just before someone "unwanted" is about to enter it and sometimes it allows them to go into the orifice. Sometimes it just stays on or closes for no apparent reason. It's a complete mess in what's otherwise a decent universe.
 * The zat guns too, although in this case it may be a subversion since they just kind of lost an ability. The rule at the beginning of the TV series was: first hit stuns, second hit kills, third hit disintegrates. It was an excuse to get rid of any inconvenient bodies that they didn't want to keep track of. The creative team eventually decided it was stupid and the ability just kind of vanished. This was lampshaded in episode 200.
 * In Doctor Who, the Doctor's Sonic Screwdriver can open doors, hack computers, do medical scans, activate machinery, cause weapons to malfunction and even reassemble barbed wire amongst many other things despite having no form of display or feedback. However, it can't do anything against "deadlock seals" or...wood. Maybe it was created by the original Green Lantern?
 * Another example would be the Doctor's TARDIS. Exactly what she is capable of doing, not doing, and the like varies as the plot demands.
 * Buffy the Vampire Slayer. Sunlight burns nameless mook vampires faster then the recurring ones.
 * For that matter, Buffy can hit a Mook vampire anywhere in the torso and he'll turn to dust, staked through the heart. It's a plot point in Season 2 when a named vampire is shot with a crossbow bolt barely missing the heart, and it's much closer than it normally takes for Buffy to kill anyone.
 * The patients on House will get sicker or better depending on how much time is left in the episode.

Toys

 * The Mask of Life did this in Bionicle. Need to get rid of a bad guy in a family-friendly manner? Mask turns him into a snake. Need to sell more toys of the same characters and give an explanation as to why they look different? Mask upgrades them into new forms. Need to seal the Great Spirit away somewhere? Put him in the mask, etc.
 * There's also Energized Protodermis, which will either transform or destroy the victim depending on their destiny.

Video Games

 * Many of the gadgets in the Ratchet and Clank fit the bill. The Dynamo from R&C3 in particular can not only make solid platforms, but can also create a swing rope to use to jump between. The Electrolizer from R&C2 can fix nigh anything, with any technical fault, just by spinning a few dials to allow a current through. Anything from a hoverbike, to a computer terminal, to a Broken Bridge.