The last letter Henry Akeley sends to Albert Wilmarth includes a strange moment where, in the course of describing the differences between Mi-Go and humans, he briefly jumps from referring to them in the third person to the first: "Telepathy is their usual means of discourse, though we have rudimentary vocal organs...". It's easy for a reader to miss the first time around (or dismiss as an error that slipped through editing), especially since Wilmarth doesn't make any note of it in his narration, but it's a subtle, creepy foreshadowing that it's no longer Akeley who Wilmarth is communicating with.
