Mindbreak Trap
Instant — Trap
If an opponent cast three or more spells this turn, you may pay 
rather than pay this spell's mana cost.
Exile any number of target spells.
"Life is a maze. This is one of its dead ends." —Noyan Dar, Tazeem lullmage
Christopher Moeller
Mystery Booster 2 (MB2)
• #168Rulings
Mindbreak Trap's alternative cost condition checks whether an opponent cast three or more spells this turn, not whether those spells have resolved.
(wotc • 2009-10-01)
If a spell is exiled, it's removed from the stack and thus will not resolve. The spell isn't countered; it just no longer exists. This works on spells that can't be countered, such as Terra Stomper.
(wotc • 2009-10-01)
You may ignore a Trap's alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.
(wotc • 2009-10-01)
Casting a Trap by paying its alternative cost doesn't change its mana cost or mana value. The only difference is the cost you actually pay.
(wotc • 2009-10-01)
Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.
(wotc • 2009-10-01)
Rulings
Mindbreak Trap's alternative cost condition checks whether an opponent cast three or more spells this turn, not whether those spells have resolved.
(wotc • 2009-10-01)
If a spell is exiled, it's removed from the stack and thus will not resolve. The spell isn't countered; it just no longer exists. This works on spells that can't be countered, such as Terra Stomper.
(wotc • 2009-10-01)
You may ignore a Trap's alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.
(wotc • 2009-10-01)
Casting a Trap by paying its alternative cost doesn't change its mana cost or mana value. The only difference is the cost you actually pay.
(wotc • 2009-10-01)
Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.
(wotc • 2009-10-01)