Minecraft wiki entry for Things_not_to_do, associated text: If the player spawns the Wither in the Nether, the Wither could destroy the floor, opening up holes to lava or large caverns and thereby increasing the chances of the player falling into lava or a pit, as well as the ceiling which could result in lava pouring down from above and possibly onto the player. If the player knows the placement of the soul sand and wither skulls, they may try using the top bedrock layer in the Nether to help trap the Wither during the fight. (This is not that hard, but has disastrous consequences if done incorrectly.) Additionally, if the player defeats the Wither near a lava ocean or lake, the nether Star may drop into into the lava and be destroyed, leaving the player with no reward for the effort.