At the end of ROTJ, Luke whispers: Thank God. I'm actually not sure whether it's audible or not, but his lips are moving using those words. That was the first clue. But now, in ROTS....
Anakin: This baby is a blessing.
A blessing from who?
Palpatine: Execute order 66.
Actually really interesting and significant Lucas uses that particular number. 7 is the perfect number in the Bible. 6 is the Devil's number, falling short of 7, the ideal. And 666 is also a really bad number, being comprised entirely of 6's.
So there's some subtle symbolism here; Lucas didn't use "66" by coincidence. Any thoughts on this in general?