Is there a case to be made that Natalie missed her trigger?

That's an interesting point, since the scenario doesn't ever say Natalie, herself, did anything to explicitly “demonstrate awareness” of the trigger. However the Definition in the IPG says

Once any of the above obligations has been fulfilled, or the trigger has been otherwise acknowledged, further problems are treated as a Game Play Error - Game Rule Violation.

and the Philosophy points out the fact that

If an opponent requires information about the precise timing of a triggered ability or needs details about a game object that may be affected by a resolved triggered ability, that player may need to acknowledge that ability’s existence before its controller does.

Why should the trigger be considered Missed if Angus clearly acknowledged it?

EDIT: Formatting tags

Edited Michael Shiver (Feb. 3, 2015 02:59:00 AM)

Well done, judges! As many of you recognized, this is a Game Rules Violation. The triggers on the stack were resolved in the wrong order, which resulted in Mardu Strike Leader being returned to Angus's hand when it should have been left on the table until the Archfiend's trigger had resolved. Since very little time has passed, the head judge should authorize a rewind, which we perform by undoing Natalie's draw (randomly from her hand) and untap steps and returning Strike Leader to the battlefield. After we have done this, the game resumes as normal with both triggers on the stack, and Angus must choose a creature to sacrifice. Angus receives a Warning for the Game Rules Violation of making this error, and Natalie will be receiving a Warning for Failure to Maintain Game State for allowing it.

That covers what the answer is, but let's also briefly explore what this situation is not.

This is not a Missed Trigger. For a trigger to be missed, it must go unacknowledged by both players. Angus clearly acknowledged the trigger, even if he tried to resolve it at the wrong time, so that's out.

This is not Drawing Extra Cards. When a card is improperly put into a player's hand from anywhere, it is Drawing Extra Cards unless there is a preceding GRV (there isn't) or it is the result of resolving objects on the stack in the wrong order. The latter circumstance happens to be exactly the case here, so we know this isn't DEC.

Thanks to everyone for participating, and we'll be back tomorrow with a new scenario.

For my own education, can I get a little more clarification on why Natalie just gets a FtMGS for resolving triggers in the wrong order (one of which she controlled)?

Because of what Walker quoted from the IPG:

Originally posted by walker metyko:

From the IPG 2.6 “A player allows another player in the game to commit a Game Play Error and does not point it out immediately.”