There are a couple sequences I never noticed before in THE VOYAGE HOME, probably due to the fact that I barely watch it and they are both the same issue. When the HMS BOUNTY (!!!) is about to fly around the sun, we see a shot of the bridge, facing the viewscreen, as the sun fills the scene with bright, yellow light. Kirk then says, "... we need break-away speed, Mister Sulu," only the "Mr. Sulu" part of the line is muted, for some reason, despite our continuing to see Shatner mouthing it.
Later in the movie, there is a scene where the whales are at sea and about to get harpooned. The BOUNTY is approaching from the sky and Kirk asks to see the whaling ship, onscreen. For reasons unknown, Gillian is amazed by this, "... how can you do that?" she blurts out. But, she does it twice. There's an odd cut, just before she delivers the line in close-up, there's a group shot where she says the line, but it's muted. We actually see her saying the line twice. Nimoy should've been more careful and caught obvious instances like this, but he just couldn't be arsed, I guess ...