First UE4 Bug
…or hilarious oversight, however you want to look at it.
The issue here was that, in addition to controlling the character’s yaw (side-to-side rotation, about the vertical axis), the firstperson camera was controlling the character’s pitch (up-and-down motion). Pitch should affect only the neck and head, but here I’d forgotten that I was working with the character’s entire body, and the result is some crazy moonwalking.