1/10/2007 Guile's second ability replaces "counter [a certain spell]" with "exile [a certain spell] and you may cast it without paying its mana cost." You have the option to cast it immediately upon its exile. If you choose not to, it remains exiled and you don't get another chance to cast it. If the spell or ability that tried to counter the spell has additional effects, it then continues to resolve.
1/10/2007 A spell exiled this way was never actually countered.
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Nin Florii Frostarii Cedars