Period movies set in a particular year are usually going to have only set dressing from that one year. I remember watching Catch Me If You Can; fun flick, and also production designed with the sort of enormous budget (enormo-bucks!) that Steven Spielberg can command, so that everything onscreen looks very, deeply 1962, or 1968 or whichever year is in that movie's particular episode. But it's almost too well set-dressed: there's Leo DeCaprio, running along a street, and I can tell all the cars are the same year's models. I'm not even a car guy.
That's yet another way that movies don't connect all that much with reality, but instead heighten reality. Within, say, a 5-block radius of my apartment are cars from probably each decade since the Fifties, maybe the Forties. At times at least three decades are represented in just our five-space dirt parking lot behind my building. So if a movie set in the present day were shot in my neighborhood**, you could leave those cars, no problem. If a film crew shows up to make a film set in, let's say 1987, out go the mix of cars and in come a bunch of '87 models. No obvious anachronisms, if they can be avoided (Frank Darabont still cringes a little bit that a car in a late Shawshank Redemption scene is a model from a year later than when the scene takes place), and they'd likely err on the side of choosing just '87 models or, maybe, mid-Eighties models, too.
But I like the mix, personally, knowing that stuff a decade old is butting up against stuff that's eight decades old. Portland's a still-young city, relatively speaking, but we have rings on curbs that people could tie their horses to a century ago. (A local arts project ties toy horses to those rings now. Neat. They add a little bit of flair to the town.) The risk to the mix, however, is being cluttered and confusing, and you don't want your film to be unnecessarily confusing. (The honking huge anachronism sometimes can't be avoided, like the truck that drives by in the background in the film Gettysburg. But nowadays, of course, the effects people could probably digitally erase said truck. The Fellowship of the Ring had a scene where the glint of a passing car was visible in one shot, so Peter Jackson had that erased for the extended and home video versions of the film.)
Still, I do prefer what Catch Me If You Can did to what The Wedding Singer did, which is "stick all of the first half of the Eighties in a blender." That was kind of lazy writing, the sort of lazy writing perfected (so to speak) by Friedberg and Seltzer in the _______ Movie Franchise of Doom: just have a reference and it will be funny! Yeah, I don't think The Wedding Singer holds up all that well for other reasons, too. (Confession! I laugh like a hyena at The Waterboy, which is purer Adam Sandler madness. "And I like Vicki Vallencourt and she likes me and she showed me her boobies and I like them too!")
Another silly example: the slight but cute flick Thirteen Going On Thirty. Its 1987 is heightened, certainly, for comedic and style purposes, but it got a lot of details right. I know, I was around back then (in junior high in Northern Virginia). Yeah, I was wearing Hobie and Ocean Pacific t-shirts and even, for a time, teal Chuck Taylors. Plus I was definitely perfecting The Art of the Crush, noticing the girls (in Esprit clothes, for example, dear God how do I remember this stuff) and thinking They're purty. The film's one big anachronism was deliberate: the songs and MTV videos the young versions of the characters like are from 1983 or so, because the filmmakers felt 1983's songs and videos were more immediately iconic and recognizable than 1987's. So they're hearing Michael Jackson's "Thriller" and Talking Heads's "Burning Down the House" instead of U2's "With Or Without You" or INXS's "Need You Tonight/Mediate" or DJ Jazzy Jeff and the Fresh Prince's "Parents Just Don't Understand." Still, as a witness to the Eighties, I saw that they'd done their research and got pretty close to lifting up a chunk of 1987 and putting it in front of cameras in this decade. (Garth Jennings's Son of Rambow is supposed to be even better in that department. And Donnie Darko re-did 1988 well, too.)
Napoleon Dynamite does its own style quirk, in that it's supposedly set in this decade but the music and fashions are either Eighties or Eighties-influenced. There is a kind of time-warp in more depressed or rural areas, like the Idaho small town in that film: even today, when media connect us so thoroughly to What's New, more older stuff lingers in such places. I could see that in Hermiston, Oregon when I lived there (heck, Hermiston still had a drive-in diner; I ate there my first day visiting the town). The makers of Napoleon Dynamite exaggerated that, for effect.
The effect is what's important. Filmmakers and other artists should be concerned with effect: what's effective? What's interestingly effective? Is "reality" important to your work? Is verisimilitude, where it feels real, important? It's one reason why Blade Runner holds up even with all of the defunct businesses being advertised in a Los Angeles that'll never exist: you buy the world. You accept it and go on. That's a rare case where "mixing up a bunch of things" works, with futuristic spinners flying by the Bradbury Building which was old when Harlan Ellison used it in The Outer Limits' "Demon With a Glass Hand," because you can feel the history of Blade Runner's L.A. It's almost entirely invented, but it feels like real history. Real, interesting history. And that's effective.
* I've already thought this before, but listening to KINK FM's one-year-a-day programming thing that it's doing to celebrate 40 years on the air is making me think this more.
** There was a film shot near my neighborhood! It's still unfinished (the special effects are being done slowly), and it's called Flesh Of My Flesh, and it was shot a few years ago in and around Portland and Wilsonville. Zombie flick, so there's blood and guts at the link. Anyway, the film shot in some of the industrial area some blocks from my place. You probably won't see my building in any of the shots.