Brain Maggot

• WWizards of the Coast2014-04-26
  Official Ruling
  The exiled card returns to its owner's hand immediately after Brain Maggot leaves the battlefield. Nothing happens between the two events, including state-based actions.
• WWizards of the Coast2014-04-26
  Official Ruling
  In a multiplayer game, if Brain Maggot's owner leaves the game, the exiled card will return to its owner's hand. Because the one-shot effect that returns the card isn't an ability that goes on the stack, it won't cease to exist along with the leaving player's spells and abilities on the stack.
• WWizards of the Coast2014-04-26
  Official Ruling
  Brain Maggot's ability causes a zone change with a duration, a new style of ability that's somewhat reminiscent of older cards like Tidehollow Sculler. However, unlike Tidehollow Sculler, Brain Maggot has a single ability that creates two one-shot effects: one that exiles the nonland card when the ability resolves, and another that returns the exiled card to its owner's hand immediately after Brain Maggot leaves the battlefield.
• WWizards of the Coast2014-04-26
  Official Ruling
  If Brain Maggot leaves the battlefield before its enters-the-battlefield ability resolves, the opponent will reveal their hand, but no card will be exiled.

Rulings via Scryfall