This is going to be really, really nerdy, so apologies in advance.
I’m watching “Star Trek: The Motion Picture” on IFC; I haven’t watched it in ages. They just showed the scene where the alien entity comes on board the bridge of the Enterprise, represented as about an 8-inch-wide vertical column of light, moving around and attempting to tap into the ship’s computers before it eventually abducts Ilia (Persis Khambatta).
Here’s what always annoys me in that scene. Apparently the base footage upon which they built the effect involved some film crew member wandering around the set holding a light of some sort. But even if he was normal size, he was no doubt quite a bit wider than the column of light in the final effect. And so, somehow, that part of the picture was cut out.
In any shot that pans (moves horizontally) across the set, you can see things in the background disappear behind the column of light and then reappear from the other side an inordinately long time later, revealing the fact that something’s been cut out. This is especially obvious with a sort of bulbous ceiling fixture – I’m not sure what it’s supposed to be, but it’s huge, and it pretty much vanishes behind the very narrow column of light before eventually popping out the other side. Obviously, a lot of people wouldn’t notice this, but once you notice it it’s all you can look at.