The one about Leia knowing her mother really bothered me when the prequels came around and he changed that concept to her dying during childbirth. It makes watching all the films 1-6 together pointless because of the glaring errors in continuity.

I'm sure if Lucas was ever asked about it he would come up with some explanation about Leia using the force etc. But to be honest, I think his "its all one movie" thing is a sham. Ep. 1-3 are the Star Wars films he always wanted to make but couldn't until he had the technology. For him they stand on their own and so if they don't have continuity with 4-6 he's fine with it since he doesn't care for those movies anyway. That's just a guess though.