Goblin Engineer

When Goblin Engineer enters, you may search your library for an artifact card, put it into your graveyard, then shuffle.
{R}, {T}, Sacrifice an artifact: Return target artifact card with mana value 3 or less from your graveyard to the battlefield.

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When Goblin Engineer enters the battlefield, you may search your library for an artifact card, put it into your graveyard, then shuffle your library.
{R}, {T}, Sacrifice an artifact: Return target artifact card with converted mana cost 3 or less from your graveyard to the battlefield.

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Because targets are chosen for activated abilities before costs (such as "Sacrifice an artifact") are paid, Goblin Engineer's second ability can't target the artifact that will be sacrificed to pay its cost.

If a card in a player's graveyard has {X} in its mana cost, X is considered to be 0.