Shakespeare's plays were written not by him.....but by another playwright of the same name. Or maybe by ancient space-invaders. Or not.
Devilstower at Daily Kos is not too sympathetic to ancient astronaut theories:
It's not the idea that there may be other life in the universe that bugs me; it's the implication that people in ages past were too stupid to stack one rock on top of another. Easter Island, Stonehenge, and the civilizations of Mesoamerica all have been targeted as potentially alien-assisted, but no civilization has suffered from a greater assumption that "they just couldn't have done it themselves" than that of the Egyptian Old Kingdom.
Check the links for the concrete details.