The mother of all vfx errors to me, and something I'm assuming they still haven't fixed for the blu-ray (unless they have???), is during the Battle of Endor.
When the Death Star II fires off its superlaser for the second time, we cut to it blowing up a ship as rebel fighters fly past the camera. The camera is rolling clockwise (or counterclockwise, I don't remember) relative to the forest moon below.
The only problem is that the fleet of big ships in the background rolls in unison with the camera!!!
Whoops!
This was obviously something they forgot to think about back in '83 when they were doing all of that complicated optical compositing.