This ruling came up at Pro Tour Dragons of Tarkir, and I've seen it play out at a few SCG Opens as well.
AP controls Whisperwood Elemental and says "Go." Is the trigger missed?
Many judges say yes. "Go" passes the turn to your opponent, so clearly you are moving past the point where you should have resolved the trigger without having acknowledged it or taken the physical action. However, this position is contradicted by the official tournament shortcut from the MTR:
"The statement "Go" (and equivalents such as "Your turn" and "Done") offers to keep passing priority until an opponent has priority in the end step. Opponents are assumed to be acting then unless they specify otherwise."
Note that the shortcut ends in your End Step, not your opponent's turn. You're giving your opponent one last chance to do something on your turn. If they don't have anything to do, they can begin their turn. Where this gets weird with Whisperwood Elemental is that this triggered ability can be on the stack when you pass priority in your End Step.
Normally, a player will pass the turn in some fashion like "Go, make a manifest" or "Ready to go to my End Step." These are pretty clear situations where the player remembers the manifest trigger and is getting ready to take the appropriate action.
It is likely that a player who says "Go" with a Whisperwood Elemental in play has forgotten their trigger. However, policy now allows for players to have forgotten a trigger as long as they acknowledge it before it becomes relevant to the game. Prowess is the classic example right now. You can cast a spell, forget about prowess, but you're okay as long as you remember before the creature deals damage.
What this means here is that the Whisperwood's controller has until he or she takes an action or he or she allows the opponent to take an action that could not have been possible. Cracking a fetch in the end step? That's fine. Trigger could still be on the stack. In fact, just about anything could happen in the End Step with that trigger on the stack. There are a few minor things possible for the controller to do that make it clear that the trigger is missed. Discarding for the turn or ending some turn-based duration effect. But for the most part, it is allowing the opponent to take an action that signals the death of this trigger.
The most typical action here is untapping permanents, with drawing a card being a close second. The tricky part about this is the part about "allowing it to happen." If Whisperwood player says "Go" and the opponent untaps, is the trigger missed? Yes, unless he or she immediately points it out. "Whoa. You can't untap yet. My manifest trigger still has to resolve." If that isn't your reaction to the untap, then you've missed your trigger.