The Lost Woods With Dark World Enemies:

In the Dark World, go and stand EXACTLY where the mushroom is in the Light world(Somewhere in the Skull Dungeon). When you use the Magic Mirror to go back to the Light World,not only will you get the Mushroom but the trees will be glitched and you'll be able to fight some of the monsters that are only in the Dark World! When you leave the forest, everything will go back to normal.