Article:
D&D 4th ed: Devious Traps and Physical Puzzles.

There are no monsters in an Indiana Jones tomb, and yet they are exactly what the average Dungeon Master wants to create for their players.
Pit traps, rolling balls of stone, darts linked to patterns of floor tiles, weighted statue triggers, impaling spikes, hazardous snakes, spiders and hair raising rides on a mine cart; one can learn a lot from watching those movies.
It makes me wonder if Spielberg ever rolled a few d20's in his day.
My first offering takes it's cue from the second Conan movie.

The Mirror Monsters.
Take on ordinary large room, add a set of standing mirrors arranged around a central clear area, as the characters set eyes on these mirrors, compare their passive perception scores with a moderate difficulty class for their level, if they pass, something fishy about these mirrors has caught their attention, they can then roll an active perception check, the DC being hard for their level.
If they fail, the trap has effectively ambushed them.
If they pass, they get the drop on the Mirror Monsters and avoid a nasty surprise round of combat.
At first, all they see is themselves and the room, but then, the mirror images of themselves begin to move, stepping from one mirror to the next, merging together into one being, the other mirrors cease to reflect an image of the real characters.
When the mirror images all merge into one, they step out of the mirror and attack, one duplicate of each actual person in the room.
Fighting the evil twin!
The mirror images are the same level, with the same hit points and basic attacks as the player characters, but no other special powers or feats, other than having an armor defense that is three points higher than the player character they are mimicking.
The one great weakness, of course, is the mirrors, and smashing a mirror during combat will inflict ten points of damage to the Mirror Monsters (all of them).