Do not know if anyone has ever heard of libration. This is the phenomenon of a planet's moon oscillating in relation to the planet, even if the moon is tidally locked to the planet.
Animation Showing Luna's Libration
Here is an article at wikipedia on
libration. This article has the same animation as shown above.
Here is a good article on
observing the lunar libration zones (pdf).
Just something more a GM has to consider when constructing an alien world.
rmfr