You walk up a wheel-chair ramp, because there are no stairs, and it is simply "AHII".
You beat a rumored boss in a game and have geared up for days, and find it only to be a One-hit-KO
Or: "Your task today gentlemen, will not be an easy one, you must infiltrate this heavily guarded fortification and De-fuse and an atomic-bomb and a hydrogen-bomb..."
When you arrive, it turns out that the personell have fled by the sight of tanks approaching.
A perfect "AHII" moment!