MTG Enchantment vs instant
Say that we have two players, Alice and Bob. Assume that Bob has a German Foil Storm Crow out and its Alice's turn. Alice wants to cast Pacifism because god, that's a scary Storm Crow. So the enchantment goes on the stack, targeting Storm Crow. Now priority gets passed to Bob and say this German Foil Storm Crow holds sentimental value to Bob. He casts Blossoming Defens targeting the Storm Crow. Now Blossoming Defense is on the stack targeting Storm Crow. Priority gets passed back to Alice and she has no response. The Blossoming Defens resolves first, giving Storm Crow indestructible and hexproof. Now the Pacifism resolves attempting to target Storm Crow.
Because of:


Resolving Spells and Abilities

608.2b If the spell or ability specifies targets, it checks whether the targets are still legal. A target that’s no longer in the zone it was in when it was targeted is illegal. Other changes to the game state may cause a target to no longer be legal; for example, its characteristics may have changed or an effect may have changed the text of the spell. If the source of an ability has left the zone it was in, its last known information is used during this process. The spell or ability is countered if all its targets, for every instance of the word “target,” are now illegal. If the spell or ability is not countered, it will resolve normally. However, if any of its targets are illegal, the part of the spell or ability’s effect for which it is an illegal target can’t perform any actions on that target, make another object or player perform any actions on that target, or make that target perform any actions. If the spell or ability creates a continuous effect that affects game rules (see rule 613.10), that effect doesn’t apply to illegal targets. The effect may still determine information about illegal targets, though, and other parts of the effect for which those targets are not illegal may still affect them.

The Pacifism tries to target the Storm Crow but now that its an illegal target it just fizzles and goes to the graveyard. Bob precedes to then beat Alice's face in with a Storm Crow and win.
TL;DR the creature becomes an illegal target and the enchantment fizzles, just going to the graveyard.