If someone casts a spell and you {mana short} in response, what happens to their spell?
Here is the answer anyway. It's found in the card rulings.
Rulings
10/4/2004 Since this is an Instant, your opponent may use Instants in response to the casting of this spell. Note that such spells and abilities will be resolved before the mana pool is emptied by the Mana Short. This makes it useless as a countermeasure for spells. To use it effectively, you need to use it during Upkeep and even then it will not prevent the use of Instant spells and abilities.
10/4/2004 If you play Mana Short in response to a spell, it will have no effect on that spell since the mana has already been paid.
10/4/2004 It even taps lands that do not produce mana.
Still good in EDH vs Omnath though lol