->''"It's like they're being summoned by the autosave function!"''
-->--'''[[WebAnimation/ZeroPunctuation Yahtzee]]''' on ''VideoGame/AmnesiaTheDarkDescent''

These [[GoddamnedBats enemies]] just appear out of nowhere.

You've blasted your way through the [[TheMaze hedge maze]] populated by [[StockMonsters unspeakable monstrosities]], and after spending what seems like most of your ammo, you've finally cleared the place out. And look! Up there on the pedestal! It's the keycard you need to [[LockedDoor open that door]] you passed a while back! Looking behind you once more to make sure it's safe, you take the keycard...

...and immediately hear the footsteps of enemies running in to murder you. You could have sworn you cleared the area of [[NinjaPirateZombieRobot zombie chainsaw maniacs]] beforehand. And the area before that, too, and the only other entrance to this area is locked. So how the hell did these guys get here? They must have teleported in the moment you grabbed the keycard.

And that's what this trope is all about. Whenever you take something important like a key (or sometimes a new gun or upgrade item), the game spawns in new enemies for you to deal with, regardless of whether or not there were any enemies left in the area. Sometimes they're teleported in, possibly [[OffscreenTeleportation around the corner where you can't see them]] so the game can pretend they were there already, and sometimes they come out of hidden rooms that open up and disgorge them (which is often called "MonsterCloset" gameplay).

Anything useful tends to be SchmuckBait, especially any PlotCoupon. You have to get it and the writers know it, so they might as well punish you for doing the only thing possible. This veers into StupidityIsTheOnlyOption if you get no chance for at least taking sensible precautions. Like planting proximity explosives that don't trigger because the enemies spawn with their jaws already sunken firmly into your buttcheeks.

If the game goes into a cutscene as you approach a PlotCoupon, it's almost guaranteed that this trope will come into effect.

Non GenreSavvy players beware: If that thirty-foot-tall dragon doesn't appear immediately when you take the keycard, he's probably waiting behind the door you entered the room from. Or maybe he's waiting for you back at the local SavePoint. Or the dungeon's entrance. Or maybe he ''is'' the keycard. Or, sometimes you actually ''can'' grab the keycard without having to fight something over it. [[ParanoiaFuel Hope you don't lose any sleep over which one it is.]]

If the new enemies are dropped off by a dropship or similar it's probably a VideoGameSetpiece or ScriptedEvent. Compare ClairvoyantSecurityForce. Also compare VillainTeleportation. Compare SuspiciousVideogameGenerosity, where taking the "keycard" doesn't ''trigger'' enemies, but the nature of the item lets you know they're coming all the same. See also MobileMenace. HeWasRightThereAllAlong is a similar trope that applies to bosses. DropInNemesis is where an enemy appears and kills you without warning due to some [[YetAnotherStupidDeath stupid action]] by you.

----
!!Examples:

* The ''Franchise/ResidentEvil'' series especially loves doing this, to the point where just grabbing a bit of shotgun ammo can be sufficient cause for a pack of betentacled wolves to leap out of the bushes and attempt to chew on your throat, which is appropriate as after a while, you start to fear success, adding to the atmosphere.
** [[VideoGame/ResidentEvil4 RE4]] takes this to borderline-cheap levels, by throwing a BossInMookClothing or two in the pre-acquisition squad that you have to beat just to get the item, like Dr. Salvador, JJ, or a Garrador, then throwing in a couple more of them in the Keycard Squad. Some of these encounters, though, such as the first one with Salvador in the village, are optional; ie he won't appear if you fight off the Ganados until they are called away by the bell before going in the shotgun house. On the castle road after the truck crashes, the Ganados don't spawn at the gate unless you're [[OffscreenTeleportation facing the opposite direction]].
*** ''ResidentEvil5'' is almost as bad. Every time you grab a key or other PlotCoupon, expect to be ambushed by a horde of Majini [[DemonicSpiders or worse]]. For example, the first Uroboros boss doesn't appear immediately when you take the Furnace Key, but waits until you approach the door that the key unlocks.
* Crops up in the Silent Hill series. Things like three Hanged Scratchers dropping in suddenly when you grab the key in the sewers in the [[VideoGame/SilentHill1 first game.]] ''VideoGame/SilentHill3'' is fairly blatant about this, with fresh enemies filling the halls outside the room when you grab a key item.
* ''VideoGame/{{BioShock|1}}'' is generally pretty good about having the characters already there (or more accurately everything is spawned by {{Scripted Event}}s just before you get there.) There were a couple more blatant ones, though, the biggest one being when you grab the shotgun. The room is absolutely clear, but now you have to fight 8 Slasher Splicers with the lights out. Fortunately... you now have a shotgun.
** Occasionally, they also play this for horror. In one area, you can walk down a flight of steps into a large, nearly empty room with only one exit. There's a cabinet in the far corner. You can walk over, search the cabinet...and then, when you turn around, you notice a number of statues in the room that weren't there before. And as soon as you get close to one of them...
** Crack a safe? A Splicer is probably going to teleport into the room behind you.
** Also, Little Sisters bringing rewards literally appear out of thin air if you are near a Gatherer's Garden when they are sent in.
** [[VideoGame/BioShock2 The sequel]] is more obvious. Anytime you enter an 'optional' room, Splicers walk in behind you as soon as you start looking around.
* ''VideoGame/{{Diablo}} II'' pulls this when activating the seals to reach [[BigBad Diablo]]. Twice they have the courtesy to run in from off-screen; once they just pop up out of thin air. Of course, you're breaking [[SoulJar Soul Jars]] in Hell, so this is [[JustifiedTrope pretty darn justified]].
** Earlier in the game is the quest for the Gidbinn. When you light up the fire in the village in the forest, a small groupe of enemies will come rushing at you, including the one that drops the aforementioned dagger.
* And of course, you can't grab a keycard in ''VideoGame/{{Doom}}'' without unleashing demons from hell, hence the name of the trope.
** [=MAP16:=] Suburbs in ''Doom II'' has the biggest squad in either game - grabbing the first key brings in a stream of imps and pinky demons, spawning once every second or two, for nearly a full minute.
** This was one of the biggest complaints about ''Doom 3''. It happens constantly throughout the game, though sometimes the enemies are just hiding behind doors that spring open whenever you grab the item(s); other times, waves of them literally teleport in.
** Seen how ''Doom'' is one of the oldest games around that people still play, there are many, ''many'' level packs for it. Few among them ''don't'' unleash massive amounts of enemies whenever you pick up a keycard; most of those squads dwarf the original levels', with ''hundreds'' of enemies being spawned - sometimes all at the same time, and often involving boss-level baddies such as Cyberdemons. Most level packs don't even bother with justifying the enemies' presence, or making their appearance somewhat plausible: they're just teleported in place, replete with teleportation noise and effect. In fact, you can often hear their idle sounds from the rooms in which they're waiting for their scripted teleportation; if you've blasted apart everything in sight, and you can still hear growling from somewhere, you better break out the [=BFG=] before you grab that keycard.
* ''VideoGame/SeriousSam'' revolves around this entirely, and manages to make it fun.
** The sequel, on the other hand, goes severely overboard...
** Sam arrives in a room at one point and hangs a lampshade, complaining about the enemies being slow to ambush him.
* At least one of the ''CallOfDuty'' games had a shed-load of enemy soldiers teleport in if the player took control of a machine gun.
** ''CallOfDuty'' is particularly egregious, in that in most games, enemies will constantly spawn out of view until the player advances to the spawn point or beyond. Meaning that endless streams of enemies were the ''norm'' rather than the ''exception''.
*** In some cases the game actually complains about it in the console. The first game will note that it can't spawn because the 'player can see spawn point'.
* Likewise, nearly every game in the ''MedalOfHonor'' series pulls the same trick. Apparently half the SS is just waiting around for you to find an MG-42 before running out and getting mowed down.
** And some levels have [[RespawningEnemies infinitely respawning enemies]] after you complete an objective. Or enemies that magically teleport in [[OffscreenTeleportation when you're not looking]]. They can even be triggered by walking into an empty house. [[MookMaker Mook-summoning]] alarms sometimes automatically activate after picking up a PlotCoupon.
* ''VideoGame/MetalGearSolid'', for example, has a bit where you get on an elevator and the weight limit warning light goes off, even though Snake is by no means heavy enough (well, except maybe for the HyperspaceArsenal he's carrying around in his pockets). When you tell Otacon about this, he mentions that some optic camouflage units have gone missing. Oh no! It's a trap! The enemy soldiers decloak (actually, they just become slightly more visible, Predator-style, and you can use the Infrared Goggles to see them clearly) and you have to kill them. However, on subsequent playthroughs you can't just clear the elevator beforehand with, say, grenades or C4, or see them with the goggles, because as it turns out they technically aren't there until the game spawns them in specifically for the event. That's the very definition of TeleportingKeycardSquad, even though the game attempts to HandWave it with the aforementioned stealth unit business.
* ''Franchise/DeadSpace'' does this quite a bit, but it's done a little more subtly than most games, because there's sometimes a certain amount of delay between the trigger and the appearance of the slavering hordes of flesh-beasts; also, the monsters almost always come crashing out of the vents, so there's seldom any "how could they ''possibly'' have snuck up on me?!" questions: they got to you the way they always do, by crawling through the ducts. This means the player can sometimes think grabbing or activating an important item didn't trigger a mass invasion of a previously cleared area when he actually is just about to get swarmed.
** Another common tactic the game pulls is to have corpses lying around which only rise as necromorphs after you've accomplished something nearby. Stomping on them a couple of times while they are still down prevents a nasty surprise later.
* In ''VideoGame/{{Half-Life 2}}'' there is one memorable annoyance in Ravenholm. New zombies are constantly generated from multiple points all around the map as you kill off their predecessors. They're not there, then poof into existence. The level is otherwise quiet, until you move about one building down the street, whereupon without warning you're damn near overwhelmed by a horde including a couple poison headcrab zombies. You're left wondering, "where was all this crap when I was about 60 yards off and couldn't hear it at all?"
** There's also a scene earlier in the game where you clear through the interior part of a gate control room so you can open the gate to take your boat through. Turns out you just need to blow up some explosive barrels to set off the controls and open the lock. As soon as you do so, a troop carrier flies in near the entrance and deposits some Combine. Although they're not teleporting in, they arrive WAY too fast to have gone through all of the complex to attack you as soon as they do. Unfortunately, it's a scripted event, so the Combine will show up to attack you as soon as you enter the area with the mounted gun, regardless if you head there immediately upon opening the gate or take your sweet time.
** Also, there's a scene in the underground freeway where you get to a room, clear out some zombies, then take the elevator out. As soon as you call the elevator, though, you're beset by hordes of zombies, as the elevator takes its own sweet time clanking down to you. Contrived, yes, but it's pretty damn scary.
*** In that same room you can be zombie-free, but if your flashlight goes out, the moans start. They're actually scripted to spawn faster when your light is out.
** The ''Half-Life'' series in general seems to love this, and in the first series it's often justified, as enemies ''literally'' teleport in but sometimes they 'port in huge numbers for no apparent reason, seemingly whenever you do something relatively important or are ''trying'' to do something important, or are simply [[HoldTheLine waiting for something like an elevator]] (similar to an example above). Waiting for a teleporter to warm up? Almost ''every kind of alien you've seen'' and ''even a kind you haven't'' 'port in! Trying to turn a valve to reset damping locks? A dozen aliens 'port in! Just finished aligning the focus emitters to activate a teleport device? You pissed off the enemy gods a dozen aliens 'port in! Waiting for a teleport field to reopen? A dozen aliens wait, nope, a dozen ''Marines'' cut open a door and ambush you! Waiting for a displacement beacon to power up? Get ready to HoldTheLine against aliens suddenly 'porting in, including Manta Rays! Christopher Odd [[https://www.youtube.com/watch?feature=player_detailpage&list=PLj_Goi54wf0fpfH_Qy1UNXIiXcRjg3JEe&v=T5vGAypGdeg#t=739 puts it well]]:
--->'''Chris:''' ''[aligns the emitters]'' Well that was pretty easy, hopefully. ...I say that, [[GenreSavvy now my face is about to]] ''blow off''. ''[aliens start 'porting in all around exactly on cue]''
* ''Franchise/{{Metroid}}'':
** ''VideoGame/SuperMetroid,'' does this twice -- with the same boss (Ridley) and the same PlotCoupon (Baby Metroid) both times.
** ''VideoGame/MetroidFusion'' has a nasty habit of spawning loads of X-Parasites from absolutely nowhere whenever you grab an item or open a plot-locked door. Justified, though, in the cases of security-level doors, because you just opened up a lot of doors for other things to come in too.
** ''VideoGame/MetroidPrime'' has this when you grab the thermal visor in the bottom of a large tower. To do this, you must shut off the power to the security fields in the room, releasing a handful of Metroids from their storage tanks and alerting the Space Pirates to your location.
*** ''Prime 2'' has you get the Gravity Boost, allowing you to move normally underwater. Then you begin to head back, and the Alpha Blogg attacks you. Just in time.
*** ''Prime 3'' does this ''twice'', but it's justified in that you can see the metroids caged up until you're forced to cut the power.
* ''VideoGame/DukeNukem3D'' does pretty much like ''Doom'', with aliens instead of demons. Also, enemies spawn as punishment for killing humans.
* ''FinalFantasy''
** Many a mission and quest is based on this in ''VideoGame/FinalFantasyXI'' - you're specifically expected to select a ??? spot on the floor to get some key item, and a mob just spawn there from thin air - only after you defeat it can you select the ??? again to get the item properly.
** [[LampshadeHanging Lampshaded]] in ''VideoGame/FinalFantasyX-2'', with the Sphere Hunter credo: "Find the sphere, and the fiends appear."
* In the original ''[[VideoGame/WildArms1 Wild ARMs]]'' game, in the Photosphere, a certain {{NPC}} only shows up after you've emptied a certain treasure chest. If you've been cheating and have the max of the item (a stat apple), this person never shows up, and you'll be mystified at how to get further into the dungeon.
* Happens quite often in the MMO ''CityOfHeroes'' when a player clicks on an objective such as a crate or computer terminal. Usually the incoming enemies spawn outside the room and run in, even shouting orders to one another so that you know that they are coming. Then there are the Rikti, which will simply teleport in around you during the final mission of the Lady Grey Task Force.
* FirstPersonShooter ''[[Area51FPS Area 51]]'' is very fond of this trope. Clear out an area entirely, go to next room, activate switch, return to previous area, find it with more enemies than it had the first time.
* ''VideoGame/MassEffect1'' is generally fair about cleaned areas staying clean, but set a timed scuttling charge or any equivalent thereof and see what happens.
** Particularly blatant at the end of the Noveria mission. Once you've cleared out the Hot Labs, you can loiter around in them as long as you want without running into any enemies... but the moment you start the two-minute countdown for the neutron purge and try to get the hell out of dodge, the next room over is suddenly packed wall-to-wall with angry rachni. Things like this happen ''multiple'' times on Noveria. "Hmm, there's a storage locker, let me get it and leaOH SHIT RACHNI KILL IT!!" (The fact that rachni look like the [[InsectoidAliens mutated offspring of spiders and cockroaches]] doesn't help.) But those times never have more than one or two Soldiers; activating the neutron purge [[ThisIsGonnaSuck spawns about 10 Soldiers and 30 Workers]]. Luckily, you can just ignore them and run straight to the elevator.
** After you wipe out Fist's men and get to him at the beginning of the game, the bar where you fought the first bunch is full again on your way out. Of course, otherwise having a three minute timer to walk twenty feet would be pretty stupid.
** One particular sidequest also plays with this: when you board the ship, the radar is completely clear of any contact, but one of your squadmates will tell you right away that "we're not alone". However, it is only after you activate one particular console that an enemy will suddenly appear behind you.
* In level 4 of ''VideoGame/PrinceOfPersia2'', you get chased to the level exit by a bunch of [[DemBones sword-wielding skeletons]], which were content to rest in peace when you ran over them on your way to unlock it.
** Something similar happens in Hooktail's castle in VideoGame/PaperMarioTheThousandYearDoor, except skeletons, Dry Bones.
* ''Winback'' has similar; if you are lucky, one of them will frag himself on the Deadly Laser Traps that are all over the damned place.
* ''[[VideoGame/GoldenEye1997 Goldeneye 64]]''; despite clearing the facility of all enemy soldiers, unlimited ones will appear once you are done talking to 006. Don't stay and fight -- just set the damned charges, detonate them and run for it. Poor 006. Hope he doesn't hold a grudge.
** Watch Alec closely when that sequence begins and you'll see him [[spoiler:deliberately shooting at the wall below the door the guards come through. That specific point is where Alec implemented his plan to pull his FaceHeelTurn]].
** Another infinite teleporting keycard squad attacks at the end of Bunker 1, as well as Control.
** In ''VideoGame/PerfectDark'''s second level, a couple of enemies spawn behind you while you're decoding the LockedDoor to Sector 4. In the fourth level on Perfect Agent, one of the additional objectives is triggered this way.
** ''Perfect Dark Zero'' pulls a cheap one at the end of the first level, when you reach the top floor of the club after clearing it of mooks, only for three more with submachine guns (which are DemonicSpiders on Perfect and Dark Agent) to spawn behind you. Later levels will throw [[HeavilyArmoredMook armored]] EliteMooks with [[MoreDakka machine guns]] or [[HandCannon magnums]] into the keycard squads.
* ''VideoGame/WorldOfWarcraft'' is prone to doing this during quests, to the point where you often actually ''see'' the enemies spawn in front of you once you grab the PlotCoupon or reach the appropriate trigger point. Only rarely is this [[JustifiedTrope justified]] by the creature being a ghost or having a teleport power. Also, in {{Escort Mission}}s, you can pretty much guarantee that you'll be ambushed by magically spawning enemies at least once on the NPC's scripted path. It's such a standard element of the game that most players are used to it by the time they reach mid levels. Spawning {{Mooks}} are also a common feature of boss encounters, even when your group has completely cleared any part of the dungeon that they could reasonably have come from.
** The Worgen starting zone prevents the visible spawns of [[spoiler:Forsaken assassins]] by having several around three spawn points - no matter which way your camera faces, one of them is always '''just''' out of frame.
* ''VideoGame/TheSuffering'' has nice twists on this and many other tropes. Many times you can look through security cams, including at your own self. Sometimes you see things in the camera in that weren't there before you decided to take a look. Sometimes they are there after, sometimes they are only viewable in the camera and sometimes you spin around in horror only to see nothing whatsoever. Explainable by the enemies being supernatural. Well, most of them. Fortunately the evil CO waits until you get to the camera before executing prisoners.
* A particular mission in ''VideoGame/StarCraft'' does this. After clearing your way to your target's base, you must escort him out, this time using his base to create units and deal with the hundred mooks that spawned on the way back.
* In ''VideoGame/{{Blood}}'', this happens ''very'' frequently.
* The original ''VideoGame/{{Quake}}'' did this a lot, sometimes summoning [[GiantMook Shamblers or Vores]]. A nasty case occurs on "The Door to Chthon", where, on all difficulties, a Shambler teleports in after you receive the Gold key, and you're in a dead end with no hiding spot. Then you reach what appears to be the exit door, but then the lights come on to reveal another Shambler.
** The [[VideoGame/QuakeII second game]] has drop-in bosses, such as the Supertank that teleports in after you shut down the security grid.
* Even ''VideoGame/{{Civilization}}'' has a variant: every time a civilization moves into a new area of the technology tree, barbarians spawn somewhere on the map and attack whatever's nearby.
* In ''VideoGame/TheGodfather: The Game'', after defeating the guards of a racket track, one of them will drop a sack of cash. If you pick it up in a part of NYC still controlled by an enemy family, mobsters will spawn and start chasing you. Usually they come in a car. You will also see these if you pick up a stolen shipment, and some raids on enemy fronts may have enemies pop up to attack you InTheBack even if you made sure to clear the path beforehand.
* Happens in spades in the ''VideoGame/{{Descent}}'' series. Certain {{Mook Maker}}s or monster closets are set to activate when you cross a certain line or retrieve a key . One of the worst examples is at the red key in the first game's Level 6, which is practically impossible to beat without the cloaking device. Another monster closet is placed right at the beginning of Level 9 to ambush you when you respawn. The worst are the ones in rooms that contain nothing else significant, only set up as traps.
* In ''VideoGame/{{Halo}}'', extra enemies don't teleport or appear from nowhere, but are usually inserted from dropships or come out of locked doors, ie monster closets.
** Played straight in the first game, in the level "The Flood." Despite the facility ''teeming'' with Flood hostiles, they only show themselves seconds ''after'' TheReveal.
* In ''VideoGame/Left4Dead'', if you have to manipulate the environment in some way to advance it's a sure bet that a horde of zombies will attack as soon as you do. [[JustifiedTrope Justified]] in the case of noisy mechanical things like elevators, since the Infected are attracted to high-pitched noise; how they know to attack as soon as the rescue ship sets sail to pick up the Survivors is a bit harder to rationalize. Justified in Dark Carnival, since you are signalling with a ''rock concert''. But why are they attacking when all you are doing is collecting gas cans?
** DummiedOut lines from the helicopter pilot that rescues the players in "No Mercy", the first campaign, {{lampshade|Hanging}} this; he says he swears that the Infected can taste hope, and they don't like it one bit.
* Happens quite often in ''VideoGame/{{Hexen}}''. In fact, basic mooks periodically spawn on every level bar the very first, just to keep you entertained.
* This happens in a few quests in ''VideoGame/{{Dragonfable}}'' when you're supposed to go in and retrieve something or someone.
* ''VideoGame/SoldierOfFortune''. Enemies can even spawn in plain sight. In at least two levels of the series, an alarm will activate and summon RespawningEnemies after you complete the main objective.
* In ''VideoGame/SensoryOverload'', some levels have security cameras that trigger enemies to come out of locked rooms or [[MonsterCloset compartments in the walls]]. The door leading to TheDragon's lair at the end of the game is equipped with a silent alarm that causes a dozen security droids to come out of the walls. "Warning! Intruder!"
* Occasionally happens in ''VideoGame/StarTrekOnline''. Justified, however, in that the enemy can simply use the Transporters to beam in additional troops as needed. Conversely, there are a number of missions where it's possible for the player to beam out without fighting the reinforcements, making the TeleportingKeycardSquad optional in those cases.
* ''VideoGame/CastlevaniaPortraitOfRuin'' has you grab a Gear which you need to put in a place at the clock tower. As soon as you take it, Legion appears in the room.
* In ''VideoGame/TheLegendOfZeldaSpiritTracks'', every time you get a big key in the central dungeon, giant disembodied hands appear out of the floor. They will move towards the Phantom carrying the key on sight, can force the Phantom to drop the key in one hit (they will then return it to its original location), and are positioned in the most inconvenient places possible for the player. Kill them? They'll just respawn.
** Then again, given the [[GoddamnBats history of disembodied hands in this franchise]], this isn't that surprising.
* ''VideoGame/ParasiteEve2'' does this after a plot advancing cut scene or when you take an important key item. Unlike the previous game, this game uses PreExistingEncounters where there are enemies spawned in a set amount of rooms and stay gone when you clear them. When the game invokes this trope, the rooms you just cleared are now hot zones again. [[TropesAreNotBad Not a bad thing if you plan to level grind or get more loot from monsters.]]
* In ''VideoGame/GodOfWar'', every time the player acquires a new ability, he is ambushed by {{mooks}}, of whom he can try out his new ability on. Justified as every enemy in this game arrives straight from hell, and come rushing at you from portals of dark smoke in the ground no matter what the situation.
* Unlike its realistic predecessors, ''VideoGame/RainbowSix: Vegas'' uses this rather blatantly. On a few points, it's possible to see them pop into existence if you order your AI henchmen ahead or equip the thermal goggles. In most other cases, you'll notice it from the fact that there's nothing in the large room untill you move two paces forward, at which points a smoke bomb will be dropped in the middle of the room, or enemies appear from 6 different doors and 3 ropes from the ceiling. And let's not get into the few HoldTheLine sequences.
** Same for ''Rainbow Six 3'' (at least the console versions) and ''Lockdown''.
* As [[WebAnimation/ZeroPunctuation Yahtzee]] noted, in ''VideoGame/AmnesiaTheDarkDescent'', no matter how good a job you've done staying hidden, solving a puzzle usually means you'd better start running from [[FanNickname Mr. Face and Mr. Tall]].
* Happens in ''VideoGame/WillRock'', though usually monsters will pop out of the sky or out of the ground with a [[HellIsThatNoise distinctive sound]].
* One of quests in ''VideoGame/{{Gothic}} II'' involves [[WhatTheHellHero opening the gates of a besieged castle]]. Cue a horde of orcs materializing at the gates, even though by that time you usually have slaughtered the whole orc army surrounding the castle.
* In the original ''VideoGame/{{Ys}}'', after grabbing a certain treasure in Darm Tower, the statues in the room spring to life as deadly DemonicSpiders. Also, some bosses are spawned by attempting to open the treasure chest in the room.
* Happens in ''VideoGame/UltimaVIIPartII: Serpent Isle''. When you fight the goblin king and drop his health to a certain amount, a small army of goblins will appear and attack, even though you may have cleared out the rest of the village beforehand.
* ''VideoGame/FalloutNewVegas'' uses this on a few occasions. One NCR quest has you tracking down a missing squad carrying supplies. You eventually find the squad, slaughtered, and when you pick up the supplies, several legion soldiers appear out of thin air and attack you.
** The Raided Farmstead appears empty, until you enter the camping trailer and trigger a raider ambush.
** In the Broc Flower Cave, after you take out several deadly [[DemonicSpiders Giant Rats]] and approach the room with the [[DiscOneNuke Ratslayer]], about five more spawn behind you.(They appear too quickly to have come up from the pit, and always when you reach the room's entrance)
** And again in Silver Peak Mine, as you're heading up the corridor to the Legendary Cazador's chamber, a swarm of the wasps spawns out of thin air behind you.
** If you have a negative reputation with either the NCR or the Legion, assassin squads will spawn. Unlike ''VideoGame/{{Fallout 3}}'''s hit squads, these are not {{random encounter}}s; they always appear at the same locations. One of the worse scripted encounters happens on the road between Nelson and Ranger Station Echo, where a group of Legionary Assassins teleports in around the corner the moment you enter the pass, and they are likely to kill Private Renolds, resulting in failure of his quest. During said quest, after you exit the mine where the NCR hostages were held, another squad of Assassins spawns.
** Vault 34 is very bad about this. It's filled with enemies that are all plainly visible (at least on radar) but are only released in scripted events. Head down a water-filled corridor, ghouls are there when you come back. Pop open a door, more ghouls behind you. Combine that with the annoying fact that companions only auto-heal when there are no enemies in sight, meaning they're ten times more likely to die than in a normal situation.
** ''Lonesome Road'' has a particularly vile instance of this trope. If you walk into a certain RV, a Deathclaw spawns on top of it to block your escape (although it's not so much 'randomly appears on top of' as it is 'implied to have dropped down from a higher highway road and used the RV to cushion the fall'). Mind you, ''Lonesome Road'' Deathclaws are even tougher than normal ones at higher levels, so this is practically a death sentence to the unprepared. Towards the end of the Divide, in a cavern containing two nuclear warheads, the [[InescapableAmbush entrance collapses]] and a [[KingMook super-Deathclaw]] named Rawr, who [[DeadlyLunge leaps clear across the room]] and [[OneHitKill kills in one hit]], spawns in the cave.
** ''Old World Blues'' isn't even modest with how blatant or often it does this. Even if you're absolutely sure everything in a kilometer radius is dead, if you pick up one of the unique ([[InfinityPlusOneSword and incredibly powerful]]) weapons in the overworld, you'll spawn at least three mooks literally ten feet away from you, sometimes appearing out of thin air in plain sight. Which weapons count is also pretty blatant, as they don't appear with the usual weight and value tags (mostly to keep NPC's from picking them up before you do). Dr. Mobius also sends a swarm of Robo-Scorpions after you almost every time you retrieve a MacGuffin. Another cheap incident is when you kill the Master Trauma Harness in X-12, only to have four more Trauma Harnesses wielding heavy weapons teleport in above (or in front of) you.
** ''Dead Money'' does this after activating the Gala Event; despite the Courier having killed most of the Ghost People throughout the villa, the event triggers the spawning of countless more.
** ''Honest Hearts'' has an inversion, where after you've collected all the items necessary to help the tribals leave Zion Canyon, all of the wildlife in the canyon ''de''-spawns, leaving you to face nothing but White Legs and their mongrels. Very fortunate, since the wildlife consisted of [[DemonicSpiders everything you dreaded in the Mojave]] but now [[UpToEleven giant-sized and even deadlier]], especially since for the final stretch you're allowed to neither fast-travel nor dump off any excess loot holding you down into the last companion's inventory.
** ''VideoGame/{{Fallout 3}}'' has this for one of the Super Mutant Behemoth encounters; you run across a wrecked train containing a cart cage with a teddy bear. Open the cage, and the bear waves its arms, then the Behemoth appears on the eastern horizon.
** After you retrieve the medicine from the Super-Duper Mart in the Wasteland Survival Guide quest, a new group of Raiders enters the building, discovering what you did to their comrades.
** In ''Point Lookout'', after installing the Cogwave Jammer on the Ferris wheel, about ten hostile Tribals immediately materialize in plain view.
** Justified in ''Mothership Zeta'' with the use of actual teleporters.
* In ''VideoGame/SyphonFilter'', you can actually see mooks "teleport" in on your EnemyDetectingRadar. The Washington Park level is especially blatant with this. Once a CBDC agent begins disarming a bomb, the enemy is immediately all over you like flies on a carcass, and several more squads teleport in while you're {{backtracking}} from defusing the first few bombs. At least justified in the second game's first level with enemies parachuting in. Another irritating example is the Krivorozhstal Mill mission in ''Omega Strain'' while escorting the mill workers. This can even happen in stealth missions, increasing the risk of dead bodies being discovered, e.g. in Ivankov's Hometown, after taking out the guards by the hostage and approaching him, another guard is scripted to approach just out of view. Taking this one out spawns yet another coming from a side alleyway.
* ''VideoGame/CaveStory'' drops minor {{Boss Battle}}s on you when you get the Gum Base and, later, the Medicine. There's also a SkippableBoss battle that only happens if you open a conspicuous chest. Also, when you get the [[InfinityPlusOneSword Infinity Plus One Gun]], ''many'' more enemies are added to the area you have to backtrack through to give your new weapon a workout.
* Enemy spawns in the ''VideoGame/{{Turok}}'' series are, not surprisingly, often triggered by grabbing a level key or other item.
* ''VideoGame/{{Xenoblade}} Chronicles'' does this with certain quest items, but it's less often mooks and more frequently a hideously overlevelled BonusBoss you'll have to game back half the game later to stand a chance against.
* ''VideoGame/FatalFrame'' does this on occasion, though in this case it's {{Justified|Trope}} because 1. Ghosts are well-known for randomly appearing out of nowhere anyway, and 2. You don't actually ''kill'' enemy ghosts... just drain their energy until they can't manifest themselves, which they eventually recover from.
* ''VideoGame/SystemShock'' does this unapologetically. SHODAN warns you that if you enter a certain room (containing the first floor's computer nodes), she'll send mooks to tear you to pieces. You do, and she does. This still exists in ''System Shock 2'', but only to a lesser extent.
* ''VideoGame/DeusExHumanRevolution'' usually avoids this with one notable exception, the picus building. Even if you knock out /murder everyone in the building several guards will appear when you [[spoiler: summon the Funicular]]. Before this you can block every door [[spoiler: before the room 404 ambush]] and the guards still get in.
* The mission "Vertical Bird" in VideoGame/GrandTheftAutoSanAndreas. If you don't blow up the other two Hydras before stealing the third, three of them will chase you. If you do blow up those two, a single one will chase you anyway. And if you are fast enough to get on the deck and destroy the Hydra that's taking flight at that moment (wich one would assume is the one that chases you later), at least one Hydra will appear out of nowhere and chase you anyway.
* The 1981 ArcadeGame ''Venture'' by Exidy presents an early example of this: each room had a treasure guarded by enemies or traps, but the Two-Headed Room appeared to have no enemies until the treasure was grabbed.
* ''VideoGame/ReturnToCastleWolfenstein'' is dirt cheap with this trope, such as dropping in enemies the moment you hit a switch to open a gate, etc.
* ''VideoGame/MegaManLegends'' averts this trope when retrieving the first large refractor. Mega Man is even confused at the lack of traps or enemies. [[spoiler: It's played straight after retrieving the second one, though.]] It should be noted that his confusion is not so much the result of being GenreSavvy, but because of the prior experience of having triggered a trap when he retrieves a large refractor during the prologue.
* ''VideoGame/PN03'' ambushes the player off-guard by literally teleporting in additional robots as they advance after clearing out the initial enemies in a room. The mission with the {{Self Destruct| Mechanism}} EscapeSequence has mooks teleported in on your escape route.
* MirrorsEdge actually averts this. Enemies who aren't directly in your path anyway spawn a distance away from the room where you meet each other. Slow players will find an ambush, average players will run into something like this trope, skilled players making good time through the stage will have been and gone before pursuers show up.
* ''[[VideoGame/{{X}} X3: Terran Conflict]]'' does this once in the Treasure Hunt plot. Reaching the third clue causes a squadron of [[AttackDrone fighter drones]] to spawn and attack, which can be a pain in the ass to get rid of depending on what you're flying.
* ''VideoGame/GhostRecon: Advanced Warfighter'' and ''Future Soldier'' are as blatant as ''Rainbow Six: Vegas''; most enemy groups don't appear on the map (and therefore cannot be seen with a UAV) until the script calls for them to to be there. Sometimes, you can see them materialize in plain sight.
* ''VideoGame/{{Descent}}'' generally justifies this by having {{Monster Closet}}s open when you pick up critical items. If the walls are of the disappearing sort rather than the "hidden door" sort, it's impossible to know they're there beforehand. But the ultimate example is the Thief-Bot. He actually doesn't have the ability to teleport and appear out of nowhere, but it sure ''seems'' like he does! In truth, the Thief is just really good at exploiting secret passageways and using his extremely high speed to catch the player off-guard.
* In ''NinjaGaiden/Black/Sigma'', enemy ambushes usually spawn when Ryu picks up a map or key item. Sometimes justified by them making a DynamicEntry or appearing unseen around a corner, but other times they just teleport into the room.
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