Rulings

• 2013-07-01
  Clone's ability doesn't target the chosen creature.
• 2012-07-01
  You can choose not to copy anything. In that case, Clone enters the battlefield as a 0/0 Shapeshifter creature, and is probably put into the graveyard immediately.
• 2012-07-01
  If Clone somehow enters the battlefield at the same time as another creature, Clone can't become a copy of that creature. You may only choose a creature that's already on the battlefield.
• 2007-07-15
  If the chosen creature is copying something else (for example, if the chosen creature is another Clone), then your Clone enters the battlefield as whatever the chosen creature copied.
• 2007-07-15
  If the chosen creature is a token, Clone copies the original characteristics of that token as stated by the effect that put it onto the battlefield. Clone is not a token.
• 2007-07-15
  Any enters-the-battlefield abilities of the copied creature will trigger when Clone enters the battlefield. Any "as [this creature] enters the battlefield" or "[this creature] enters the battlefield with" abilities of the chosen creature will also work.
• 2009-10-01
  If the chosen creature has in its mana cost (such as Protean Hydra), X is considered to be zero.
• 2012-07-01
  Clone copies exactly what was printed on the original creature and nothing more (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.