What is/causes that green oval that occasionally flashes on films?

Ever notice? Sometimes when you’re watching a movie on the big screen, a bright green oval will momentarily appear somewhere in the picture. It never lasts very long – maybe the duration of one or three frames. Always an oval, always green. What is that? What causes it? Presumably it has something to do with the chemical composition of celluloid.

And sometimes you see a vertical line running through the whole picture – also green – which can last a lot longer.

Do you mean the oval that is in the top right corner of the screen? Because that’s a ‘we’re nearly at the end of the reel’ mark, to tell the projectionist to have the next reel prepared. It’s an outdated concept for multiplexes that have the whole film on a single platter, but is still required for films in smaller theatres.

It’s oval because of the anamorphic lens stretching the circle wider.

However, if you’re not talking about that, then I think it’s probably just a dirt artifact.

Notice that the reel change marks actually flash twice within a few seconds of each other. The first set of marks is a signal for the projectionist to start the other projector’s motor, and the second set of marks is for the projectionist to close the shutter on the first projector while opening the shutter on the second projector.

On black and white films, the marks were stamped with black ink to the negative, which then would appear white on the print.

Or you may be referring to coded anti-piracy identifiers.

No, it doesn’t look anything like that.

Something like this? That’d be the changeover cue the previous posters were talking about. But I don’t think it’s usually green.

It’s actually black-and-white, but it can appear as a variety of colors, depending on the color(s) in the picture surrounding it, due to an optical illusion.

That’s the sign of a careless projectionist and/or a damaged projector. Whether it was bad handling, or a burr in the projector, it’s nothing more than a long scratch into the magenta dye layer on the film.

The magenta layer is usually the closest to the surface on most film, and in the subractive color scheme, the absence of magenta gives green. A deeper scratch that goes into the middle cyan layer will be bluish. A really bad scratch that’s gone through all of the dye layers will be clear.
clear film base

I believe you are talking about the “cigarette burn” (i.e., the reel change indicator others have mentioned)

A name from Fight Club which isn’t actually used in the industry, Wikipedia tells me. (But I like the name (and, for that matter, Fight Club) enough; I hope it takes over)

Is it unusual to be able to perceive those as they whiz by? 'Cause I see 'em all the time and they drive me batsh!t.

I can also hear sounds meant for bats and nervous, over-bred dogs, too, BTW.