One card (Circling Vultures) has the ability “You may discard Circling Vultures any time you could cast an instant.” Doing so is a special action. A player can take such an action any time they have priority.
May 3, 2019 · War of the Spark
Resolving spells and abilities may instruct players to make choices or take actions, or may allow players to activate mana abilities. Even if a player is doing so, no player has priority while a spell or ability is resolving. See rule 608, “Resolving Spells and Abilities.”
January 25, 2019 · Ravnica Allegiance
Resolving spells and abilities may instruct players to make choices or take actions, or may allow players to activate mana abilities. Even if a player is doing so, no player has priority while a spell or ability is resolving. See rule 608, "Resolving Spells and Abilities."
May 1, 2014 · Journey into Nyx
Resolving spells and abilities may instruct players to make choices or take actions, or may allow players to activate mana abilities. Even if a player is doing so, no player has priority while a spell or ability is resolving. See rule 608, “Resolving Spells and Abilities.”
September 30, 2011 · Innistrad
Resolving spells and abilities may instruct players to make choices or take actions, or may allow players to activate mana abilities. Even if a player is doing so, no player has priority while a spell or ability is resolving. See rule 608, "Resolving Spells and Abilities."
June 17, 2011 · Commander 2011
Resolving spells and abilities may instruct players to make choices or take actions, or may
April 1, 2011 · Duel Decks: Knights vs. Dragons
Resolving spells and abilities may instruct players to make choices or take actions, or may allow players to activate mana abilities. Even if a player is doing so, no player has priority while a spell or ability is resolving. See rule 608, "Resolving Spells and Abilities."
February 4, 2011 · Mirrodin Besieged
Resolving spells and abilities may instruct players to make choices or take actions, or may allow players to activate mana abilities. Even if a player is doing so, no player has priority while a spell or ability is resolving. See rule 608, “Resolving Spells and Abilities.”
October 1, 2010 · Scars of Mirrodin
Resolving spells and abilities may instruct players to make choices or take actions, or may
July 16, 2010 · Magic 2011
Resolving spells and abilities may instruct players to make choices or take actions, or may allow players to activate mana abilities. Even if a player is doing so, no player has priority while a spell or ability is resolving. See rule 608, "Resolving Spells and Abilities."