The illusion creatures printed in Magic 2012—Phantasmal Image being the most notable—have a common characteristic.  They share a triggered ability that reads: “When [this creature] becomes the target of a spell or ability, sacrifice it.”  Once one of these creatures is targeted, its sacrifice ability will trigger and that ability will go on the stack before any player gets priority.  The illusion will be sacrificed when that ability resolves, regardless of what happens to the spell/ability that targeted the illusion in the first place.

For example, if a player controls a Phantasmal Image that is a copy of Insectile Aberration, and her opponent plays a Vapor Snag targeting the copy, she won’t be able to save the frail insect by countering the Snag or by changing the target with Spellskite.  You can counter the spell or change its target, of course, but the illusion is still going to get sacrificed regardless.

Today’s Rules Tip written by
Jorge Pinto, Level 1 Judge from Santiago, Chile