Abbott and Costello Meet the Keystone Kops features vehicles made between 5 & 15 years after the Kops’ final film (1917). The Kops’ stunt truck used in the film is from at least 1928!
No, that’s Robin Hood (1991) with Kevin Costner.
(Which was probably about mid-way between Robin Hood (2010) and Robin Hood (2018) in how realistic and historically accurate it was trying to be.)
Frankly, I’ll give them that one. Asking anyone other than Rami Malek to wear uncomfortable false teeth is too much. 
And that applies to contact lenses and smallpox scars, too. Tattoos, though…cover them up!
Bad teeth can be fatal if not properly treated.
A lot of people thought “The Hurt Locker” was a documentary because it was shot on videotape, but there was a blooper that made that impossible. Early in the movie, a soldier is defusing an IED, and “There’s an Iraqi with a video camera; he’s probably going to put this on You Tube.” The movie is set in the fall of 2004, and You Tube didn’t debut until early 2005.
Also, in the movie “127 Hours”, the actor’s facial hair never changes.
In Gladiator, there’s a scene where Russell Crowe walks through a field of wheat. You can clearly see that the ripening wheat comes up to his waist. This was impossible in ancient Rome, as the short, semi-dwarf varieties of wheat were only invented in the mid-twentieth century. Ancient wheat grew to head height or taller.
A historian on Youtube goes through this scene and shows that nearly everything in it would be very different in Roman times, from the wheat, as you mentioned, to the road, lack of fences or barriers, etc.
If I’m understanding the OP correctly, I think that falls into the same category as jet contrails in outdoor fantasy/historical settings (intrusions caused by circumstance, not active mistakes like getting the wrong outfits from wardrobe department and dressing everybody decades or centuries away from the supposed setting).
Back in the 1990’s, I read two short stories (I can’t remember the titles, but one was in Good Housekeeping magazine), both set in the 1800’s, both featuring women singing the hymn “The Old Rugged Cross”, which was written in 1912.
The kilts in Braveheart are an obvious one (though it has to be said I never actually watched and said “Wow, they are wearing kilts, how anachronistic!”. its been commented on in pretty much every discussion of the movie I’ve ever read or heard though).
I’ve been rereading the Foundation series lately and while it’s easy enough to forgive the things Asimov didn’t get quite right (space ships, computers, etc…) it was was extremely jarring to read “Hari Seldon went to the park, purchased a paper from a newsstand and sat on a bench”. It’s weird to think that something as simple as a newspaper will be ancient history in a few decades.
I remember an episode of That 70s Show where Eric was holding a Millenium Falcon toy. Only it wasn’t the vehicle toy from the 1970s, it was the very obviously different figure carrying case from 1997.
Though that is by far from the most anachronistic thing in that movie IMO. The battle scene is the most egregious one to me. Seeing as so much time and effort was spent on apparent authenticity, why not actually try and recreate a battle in Imperial Rome, rather than a dumb comic book rendition? (for the same reason the ridiculous battles in 300 don’t actually bug me, as that is meant to be a comic book style retelling of the story).
They hired a professor of classics from Harvard, who’s a respected world-class expert on gladiators and gladiatorial re-enactment, as a technical consultant on the movie.
I’m not sure if I’ve ever posted her story on SDMB. It’s long, but it’s worth reading.
They totally ignored everything she ever said, and every bit of advice she ever gave, even things that were very easy to do, and that wouldn’t have cost them extra.
She asked for her name to be removed from the credits, due to damage to her reputation – but they included her name in the credits anyway.
To say that Gladiator is highly inauthentic is an understatement, not just in details, but in major things.
My nephew will go on and on about the inaccuracies in 300, but the only one that bothered me is the Spartans talking about the virutes of democracy. 
Here’s another Trek example—Spock occasionally uses what looks like a circular slide rule. I learned that this is an E6B flight computer.
Western movies will teach you a little bit about what working cowboys wore in the 1880s. They will teach you a lot about what rodeo cowboys wore in the year the movie was filmed.
According to an acquaintance of mine, in most Westerns filmed after World War II, nearly everyone uses a style of saddle that was not invented until the 1920s.
Short or Great kilts? Great kilts are period, although modern clan tartans didn’t exist back then, really.
I doubt that.
After those wars, Sparta became more and more democratic- or rather republican. But at the time of 300 they had a elected body, and critical long reach decisions were debated in that body, who distilled the choice down to two, and then the Citizens voted on which one.
But yes, at the time of the 300, Sparta can be best described as a Oligarchy with a few democratic features.
When Pompeii was uncovered they found streets with ruts where the wagon wheels had cut regular pathways, and corners which had been chipped into arcs from the wheels making sharp turns. There were also large stones laid in between the ruts so that people could hop across streets without setting foot in the river of sewage that was flowing along them. Hard to think of any Roman movies which shows anything but nice paved streets for the chariots.
Also hard to think of Roman movies where the actors didn’t have current hair and makeup. And today most male actors seem to have shaved chests (and sometimes arms and legs) no matter what period the movie is set in. Female actors haven’t had arm or leg hair for decades, of course. (All references are to American movies.)