Danger Zones are stage hazards making their debut appearance in Smash Run in Super Smash Bros. for Nintendo 3DS and also appearing in Super Smash Bros. for Wii U and Super Smash Bros. Ultimate. Characters who touch them will either take damage or be instantly KO'd without ever passing the blast lines (although in the case of the former, the target will die instantly regardless of accumulated damage, but at lower percentages in the latter, they will survive).
Danger Zones make their debut in Super Smash Bros. for 3DS in Smash Run. Some "Defeat Them All!" challenge rooms feature floors which become Danger Zones every few seconds, instantly KOing the player regardless of accumulated damage. Dotted throughout the main map are Danger Zones in the form of lava-colored spiked balls, which also KO regardless of damage.
In the "Run!" and "Climb!" final battles, Danger Zones are placed throughout the course. They appear as either pools of lava or as bunches of spiked balls. Their effect is the same as in the Smash Run itself: if a fighter hits one, they are instantly KO'd regardless of their current damage and are revived further back/down in the stage. Unlike in the Smash Run, however, Danger Zones disappear after being hit once, allowing all players to more easily traverse the stage.
Danger Zones make their next appearance in Super Smash Bros. for Wii U. In The Great Cave Offensive Danger Zones are placed throughout the stage, and appear as noticeable red-orange areas resembling lava. If a fighter makes contact with one, they will take 25% flame damage and heavy knockback, but if they had equal to or over 100% damage before touching it, they will be instantly KO'd. If the KO belongs to another fighter when this event occurs, that fighter gains credit for the KO. It is possible for a character to tech from a Danger Zone without taking damage and/or getting KO'd, but this is rather difficult..
Danger Zones are also found inside Master Fortress during the Master Core battle at the end of Classic Mode, more prominently during the second wave than the first. Unlike the examples above, these Danger Zones take the form of either subtle yellow outlines on the walls or as more expansive pools of acid resembling those found on Brinstar. Their Obliviate Effect also causes a One-hit KO the enemies inside the fortress. Like in The Great Cave Offensive, touching one while under 100% damage will inflict 25% damage and high knockback, but touching one while at or over 100% damage will instantly KO the player.
Danger Zones make their return in Super Smash Bros. Ultimate. Here, The Great Cave Offensive, a returning stage, has the same Danger Zones as in for Wii U. Aside from no longer dealing flame elemental damage they are functionally the same.