Falling is uncontrolled downward movement. For example, if forced movement causes a creature to occupy a square over a pit or precipice, it may fall.
Movement due to falling does not trigger opportunity actions, including opportunity attacks.
Large, Huge, and Gargantuan creatures only fall if every square they occupy is over an edge.
Catching oneself
If forced movement, such as from bull rush or another power, would move a creature over an edge, the creature may make an immediate saving throw to catch itself. On a 10 or higher, it falls prone on the last square it was on before it would have gone over the edge. Otherwise, it goes over the edge and begins falling. In either case, the forced movement then ends.[PH:284][RC:212]
Falling damage
A fall of 10 feet or more may inflict damage on a falling creature. The creature takes 1d10 damage for each 10 feet it falls, up to a maximum of 50d10 damage for a fall of 500 feet or more.
A faster alternative damage calculation for falls of 50 feet or more is 25 damage for each 50 feet, plus 1d10 for each extra 10 feet.
When a character trained in Acrobatics falls, the character can make an Acrobatics check as a free action to reduce the damage he or she takes from the fall. One-half the result of his or her Acrobatics check, rounded down, is subtracted from the falling damage.
A creature that is flying when it begins falling descends the full distance of the fall, but subtracts its fly speed from the distance of the fall for the purpose of determining falling damage. For example, a flying red dragon, with a fly speed of 8 (8 squares = 40 feet), takes damage from a fall as though the fall were 40 feet shorter, and does not take damage from a fall of less than 50 feet.
A creature that takes damage from falling falls prone when it lands.[PH:284][RC:209]