Card Rulings:Mist Wurm

OCG Rulings

 * If "Sangan"/"Goblin Zombie"/etc are used for a Synchro Summon, then their effects are manditory, and must activate as soon as the Synchro Summon is successful.
 * If "Goblin Zombie"/etc is used for the Synchro Summon" of a monster with a mandatory Trigger Effect like "Mist Wurm", then both effects activate simultaneously. They activate in the order that the player chooses, and then you resolve in reverse order.