From this thread.

Player A plays Armageddon targetting his attacker.

Player B uses Molecule Man's power to copy Armageddon, targetting his opponent's attacker.

Does that work?

no
Armegeddon says "Attacker you control" you do not control the attacker therefore you cannot use Armegeddon, you may copy Armegeddon with Molecule Man's effect, but to no benefit

Crossposting goodness..

To be more explicit.. You can copy the effect fine, and it goes on the chain with the opponent's attacker as the current target.
You cannot choose a new target because there is no new legal target.
On resolution, the target will be found to be illegal and the effect will be negated.

(Normally, an illegal target like this would be found on *playing* the effect thus rewinding the game to before the effect was played. In this case, though, the new copy rules skip that step so that the effect makes it onto the chain.. only to be negated on resolution.)

Exactly---the new copy effect can be left with illegal targets, but the new Copy section of the CRD (specifically 708.2e) tells you to follow the normal rules for resolving the effect. The second check for target legality is going to happen, even though you bypassed the first.

Okay, cool. Makes sense.