Second Guess — ruling 1

Second Guess can’t be cast unless the spell it targets is the second spell cast this turn.

Second Guess — ruling 2

It doesn’t matter if the first spell cast that turn has resolved yet or not. It also doesn’t matter how many spells have been cast after the second one, as long as the second spell cast is still on the stack.

Second Guess — ruling 3

Most spells and abilities that copy a spell create that copy on the stack. These copies are not cast and won’t count toward the number of spells cast that turn. However, a few (like Isochron Scepter’s ability) instruct you to copy a spell and “cast the copy.” These spells are cast and will count toward the number of spells cast that turn.

Second Guess : règles, fonctionnement et interactions (MTG)

Second Guess is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Counter target spell that's the second spell cast this turn.

How it works : Second Guess

Original English text — Wizards of the Coast / Scryfall

#1May 1, 2012· Wizards of the Coast
Second Guess can’t be cast unless the spell it targets is the second spell cast this turn.
No other card is named in this ruling.
#2May 1, 2012· Wizards of the Coast
It doesn’t matter if the first spell cast that turn has resolved yet or not. It also doesn’t matter how many spells have been cast after the second one, as long as the second spell cast is still on the stack.
No other card is named in this ruling.
#3May 1, 2012· Wizards of the Coast
Most spells and abilities that copy a spell create that copy on the stack. These copies are not cast and won’t count toward the number of spells cast that turn. However, a few (like Isochron Scepter’s ability) instruct you to copy a spell and “cast the copy.” These spells are cast and will count toward the number of spells cast that turn.
No other card is named in this ruling.