If a player controls both “Ancient Forest” and a Gladiator Beast monster that has attacked, at the end of the Battle Phase he can choose which effect to activate and resolve first.[1]
If “Sasuke Samurai” attacks a face-down monster and destroys it with its effect, it will still be destroyed at the end of the Battle Phase.[1]
If a monster attacks and is removed from the field [sic] or ceases to be a monster before the end of the Battle Phase, it will not be destroyed.[1]
If a monster attacks while “Ancient Forest” is being negated, but before the end of the Battle Phase the effect negating “Ancient Forest” disappears, the monster that attacked will be destroyed.[1]
If a monster attacks and is flipped face-down by “Book of Moon” it will still be destroyed.[1]
The effect of “Ancient Forest” will only destroy a monster during the turn it attacked.[1]
“Ancient Forest” will not change a monster to face-up Attack Position if it was not on the field in Defense Position when “Ancient Forest” was activated.[1]
Mentions in Other Rulings
Scrap Worm: If "Scrap Worm" attacks while "Ancient Forest" is active, you resolve the turn player’s effect first and then the opponent’s. If both are controlled by the same player, they can be resolved in any order.[2]
OCG Rulings
If a monster battles and is then flipped face-down, then the effect of "Ancient Forest" will still destroy it at the end of the Battle Phase.[3]
If a monster's attack is negated by an effect like "Magic Cylinder", then the monster will not be destroyed by the effect of "Ancient Forest" at the end of the Battle Phase.[6]
The effect, "When this card is activated, if there are any Defense Position monsters on the field, then those monsters are changed to face-up Attack Position." of "Ancient Forest" is not applied at activation - it is applied when resolving the effect, after the opponent has had the chance to chain to the effect. If "Mystical Space Typhoon" is chained to the activation of "Ancient Forest", then "Ancient Forest" is not face-up on the field when its effect is resolving as Chain Link 1, so the monsters are not changed to face-up Attack Position.[7]