And while we're on the topic of unexpected cameos, the Metal Gear review and the Anti Terror Force review both feature cameos by none other than Doug Walker, a.k.a. The Nostalgia Critic. Many jaws dropped on the days those videos were uploaded.
His fake trailer on a hypothetic My Little Pony film. He actually managed to make it EPIC.
Speaking of fake trailers, he also did one for a making-of in his second FAQ video. It's both hilarious and epic.