A stage hazard is any aspect of a stage which could pose a threat to players. Most stage hazards deal damage and knockback, although a few involve sudden movements of the stage or a panning camera that can carry a character into the blast line. Less agile characters are especially vulnerable to camera panning.
Stages with many or overwhelming hazards are typically banned in tournaments as they tend to interfere with the outcomes of matches or because the competitors only prefer aspects of a match that involve the characters. Factors usually considered when deciding stage legality usually involves how predictable a stage hazard is and how much influence it has over the match.
Some stage hazards may be extremely minute or subtle, or only apply to certain characters; For example, the balloon in Smashville may unexpectedly collide with Ness's PK Thunder, making him temporarily vulnerable or leaving him in the helpless state if he is in the air.