Traps

Traps are, as the name suggests, special events that can harm the player character when they are triggered by certain events like opening a chest or passing over a certain trapped piece of ground. On most occasions a creaky sound can be heard before the trap springs. The trap also often fires after a short delay. Traps have appeared since Diablo I, were carried over to Diablo II and have been modified in functionality in Diablo III. Some traps harm players only, while others can be used to damage enemies as well.

Diablo I
In Diablo, any interactable object (door or container of any kind) can turn out to be rigged. Upon touching it, a distinctive sound is heard, and the trap springs. Usually it releases an arrow from the wall, but can also release a random damaging spell (up to, and including, ). Non-missile spells are released from the object itself.

In a manner of speaking, barrels can be traps too, for they may explode when touched, or summon a Skeleton.

Rogue can see the traps (name of an object turns red) and disarm them with her special Trap Disarm Skill. Other classes may buy items that halve damage of traps, although effects of several such items do not stack. Alternatively, one may just try to outrun the bolt... or simply survive through the damage.

Diablo II
In Diablo II, one of the following things could happen:
 * A Poison Cloud is expelled.
 * A is cast around the area.
 * A is cast around the area.
 * A Fire Ball is cast in the direction of the nearest player or their followers.
 * A Lightning bolt is cast in the direction of the nearest player or their followers.
 * The chest or container ignites and burns.
 * One or more Skeletons spawn (only in caves/tombs/sewers).
 * One or more Zombies spawn (only in caves/tombs/sewers).
 * One or more Embalmed spawn (only in caves/tombs/sewers).
 * One or more Flying Scimitars spawn.

Apart from 'normal' traps, some special areas are also trapped:
 * In Act II, some floors of tombs, as well as the ground of the Arcane Sanctuary, are booby trapped with spikes. These traps are identifiable as dotted holes in the floor (usually multiple spikes in a row) and are triggered by standing on them. When triggered, the usual trap sound can be heard. If avoidance is not done immediately afterward, the spikes will rise up through the holes in an attempt to harm the entity foolish enough to be in their way. A successful hit will do a small percentage of the character's Life in Physical damage. Only player controlled heroes can trigger these forms of traps.
 * In Act II and Act III, some pillars or statues, or squares of floor are booby trapped with poison or metallic projectiles. The traps are triggered by standing in front of the pillars/statues, after which the projectile fires. A triggering sound can be heard, though it's played at the same time the trap springs. However, the projectile moves slowly, so it has the equivalent of a delay. Unlike spikes, your followers can trigger these traps, and will not move away even while being hit repeatedly.
 * In Act V, a special series of traps can be found in the Worldstone Keep. If a player remains stationary for some time, Baal's demonic laughter can be heard before a random elemental attack is launched on the location of the player. These attacks include poison clouds and lightning bolts and can be considered 'traps'. Presumably, Baal is watching the player's progress and punishes those that are idle.

Diablo III
Neither  nor containers trigger traps in Diablo III like they did in previous games. Of all the potential dangers to be considered a 'trap', the following environmental objects are the most likely to be considered as such:
 * Falling Tree Log: These tree logs simply fall onto the hero, causing Physical damage. Most of the time, these can be encountered wherever goatmen are present in or in Ruins of Sescheron.
 * Falling Axe: In the Halls of Agony, these axes function somewhat like the tree logs encountered earlier, with a few changes. If avoidance is not done, the axes inflict Physical damage and a minor Knockback. The trap does not simply cease to be after activation
 * Pendulum Axe: variant of the Falling Axe, only found in Sescheron. Has greatly augmented damage, enough to one-hit kill an average monster.
 * Moonseed: An unwary hero approaching this plant in the Dahlgur Oasis will cause it to expel a wave of high Poison damage.
 * Demonic Forge: Found in the depths of Arreat Crater, these forges spew forth a cone of Fire, capable of multiple hits.
 * Demonic Mine: First seen after embarking upon the battlefields between Bastion's Keep and Arreat Crater, this mine will trigger when an unsuspecting hero walks close. If avoidance is not done after triggering, the mine inflicts Fire damage and Knockback. In the Battlefields of Eternity, smaller versions of these mines are found, which do not inflict Knockback.
 * Pillar of Fire: a pillar of demonic origin tipped with an orb of flame. Upon approaching it, the orb detonates, causing the pillar to explode for high Fire damage in a small range.
 * Furnace Vent: come in many varieties, but the idea is periodically unleashing a torrent of flames either vertically or horizontally. Mostly encountered in the Plague Tunnels, Repository of Bones, Keep Depths and Halls of Agony. Some are destructible.
 * Fortress Freezing Gate: Seen in the Pandemonium Fortress, these gates will inflict minor Cold damage and Freeze if anyone were to attempt to cross it while it emanates cold air.
 * s of Pandemonium
 * Floor Spikes in Ruins of Sescheron. When a lever is activated, the entire room is impaled with spikes, dealing damage to players and monsters alike.

One can (and should) lure enemies to these traps: nearly all traps are affected by friendly fire as well; if a Goatman were to try to cross the path of a falling tree log, but it hits the demon, it would suffer Physical damage. A, life permitting, can also intentionally try to set a trap off; if he/she suffers damage from the trap, Fury can be generated. Cursed Chests and Cursed Shrines will trigger events, but this is expected from these objects, and should not be called "traps".