Empty Room Until the Trap

This is when a room looks empty of anything significant, and it's not. There's a well hidden trap that will set off if the hero either tries to leave or sets off another trigger, usually by the standard look-see or an Event Flag.

If the hero is lucky, it will just be a monster to fight. If the hero is unlucky, then Have a Nice Death.

Common in adventure video games and Tabletop Games.

Compare He Was Right There All Along, Empty Room Psych (which this trope often leads to), Suddenly Harmful Harmless Object, Teleporting Keycard Squad, Drop in Nemesis (where the trap results in instant death).

Film

 * Virtually every room in Cube, although the characters soon catch on to this.

Tabletop Games

 * Dungeons and Dragons has a monster that pretends to be a ceiling, and a monster that pretends to be the floor, monsters that cover the walls and almost invisible jelly monsters in the size of a usual Empty Room. Combine them all to create the infamous Room of Death, which appears empty until all of it tries to eat you.
 * Put the monster that looks like a treasure chest in the middle and the monster that looks like a door on the far wall, and you've got the Room of Vile Death. Put a pool of the monster that looks like water in the corner, and you get stuck with the whole pizza bill for the night.
 * And of course there's not a DM alive who has never used the Paranoia Fuel aspects of this trope to get their players worked up over a genuinely empty room. It's fun to watch them poke every square inch of a 10x10 room with a stick, swing their swords in a panic at casually mentioned details like some moss in the corner, and maybe even waste spells trying to divine the room's purpose or buff themselves up for a fight that never happens.

Video Games

 * The catacomb pitfalls in King's Quest VI.
 * The bathroom in Silent Hill 3.
 * Later on, the "bloody mirror room" in the alternate hospital.
 * Take the dagger out of the fridge in Silent Hill 1 without first locking the door with the Ring of Contract, and an unknown monster comes out of the fridge and eats you.
 * The "Monster Houses" in the Pokémon Mystery Dungeon series, which appear empty until 30 or so enemies drop from the sky the moment you set foot in them.
 * Unless you scan for items, since such trap rooms tend to have abundance of loot.
 * Also at the beginning of secret episode 4 in Explorers Of Sky (Here Comes Team Charm), a Graveler walks into a room with a treasure chest and statues of Lopunny, Gardevoir and Medicham, long story short the statues come to life and take the treasure for themselves, (info can be found here).
 * Such "Monster Houses" are common in Pokemon Mystery Dungeon's genre, but they are usually filled beforehand.
 * In Pokémon Diamond and Pearl there are a couple of occasions like this. The first is the ghosts in the Old Chateau which will appear out of nowhere even if the rooms look empty. The second is a small room in a cave, where the room seems empty until you leave and go back in, at which point Heatran has appeared from nowhere.
 * Left 4 Dead has these in the form of crescendo events. In most levels, there will be a room that will have a few zombies, but won't spawn any more into the map until the survivors press a button or something that alerts a horde to their location.
 * Boss rooms in Quake II and Quake IV tend to be empty of enemies until the boss suddenly teleports in. This is usually triggered by the completion of an objective, such as shutting down the security grid in II, or the Tetranode in IV.
 * Tales of Symphonia uses this to remove characters from your party just before the (apparent) endgame; there are six rooms that the heroes pass through, and in each one is a trap of some sort that requires one party member to stay behind to hold open a door, hold off enemies, etc.
 * In the first dungeon of The Legend of Zelda Ocarina of Time, the boss room is empty until you think to look up and find the boss on the ceiling.
 * In the Forest Temple, the boss room is empty. Realizing this, you leave. This triggers the boss battle.
 * In the Fire Temple, you can actually leave the boss room, provided you have not jumped out onto the battle platform yet.
 * Halfway through the Water Temple, you come into a large room filled with ankle deep water and a small island with a dead tree in the center. There's nothing particularly interesting about the room, and when you reach the other side, you find that the exit door is barred shut, and your reflection is gone. Only then will you find that one of the most fun fights of the game has been waiting, watching you from that tree.
 * It's interesting to note that if you walk backwards to the door, the boss will not appear. He will only appear if you turn the camera around so he can't be seen when he appears. When you turn around again, the boss will be standing by the tree, just waiting.
 * In The Legend of Zelda a Link To The Past, the dungeon beneath the Village of Outcasts has a room like this. The compass tells you that it is the boss's room, but there is no boss... until you bomb its ceiling and lead a certain prisoner into the light...
 * Many side rooms in the Medal of Honor series only serve to trigger ambushes.
 * Final Fantasy II has many empty rooms with very high encounter rates. It drops you off in the middle, and you'll probably have to fight a battle or two on your way out.
 * The infamous "closing wall" room (just before the throne room) in the Baron Castle from Final Fantasy IV, which has no significance until you try to walk back through it after you beat Cagnazzo, and requires the.
 * Monkey Island 2 has the seemingly empty guest room in the Woodtick Hotel. If you wait in here a while without going behind the dressing curtain Largo LaGrande will come in here and shout at you for coming in his room, and then kick you out.
 * And then later on, you set a trap for him, in the very same room!
 * Even later, you have to come in there with the voodoo doll and shock him. After this the room is really empty.
 * Trapped 5 has the first two boss rooms marked only by a preceding hallway with a question mark on the floor that says "Beware" if you examine it. You enter the room and the door locks behind you. Once you go up to the other door, then the boss appears.
 * All over the damn place in Serious Sam games.
 * Doom: See that room there? The one without monsters? With a powerup in the middle? On a pedestal? Wanna bet that once you reach for it the lights won't go out and the back wall won't open to release a dozen bloodthirsty demons?