I was joking that both were valid uses, though completely different meanings.
Merriam-Webster's Unabridged wrote:
Affect: –verb (used with object)
1. To act upon; to produce an effect or change upon.
ibid. wrote:
Effect –verb (used with object)
10. to produce as an effect; bring about; accomplish; make happen
Ergo, in the first case the "movement is not affected by terrain" would mean that the terrain does not produce a change upon the movement. The latter, "movement is not effected by terrain" would mean that the terrain does not
cause the movement. I joked that naturally the terrain does not
cause the movement since there are no moving terrain tiles in this game, making the statement "movement not effected by terrain" a valid statement, though admittedly not the statement that Jed had intended to make.