Giant Trap Door Spider

• WWizards of the Coast2004-10-04
  Official Ruling
  If Giant Trap Door Spider leaves the battlefield before its ability resolves, the ability will still resolve as normal. The targeted creature will be exiled.
• WWizards of the Coast2004-10-04
  Official Ruling
  If the targeted creature leaves the battlefield (or otherwise becomes an illegal target) before the ability resolves, the ability won’t resolve. Giant Trap Door Spider will remain on the battlefield.

Rulings via Scryfall