Characters make mistakes in stories all the time. No one thinks the detective is a moron for accusing the wrong person early in the story.

Fantasy characters also routinely spout gibberish as if it's true, because it is true in their world. No one bats an eye when a guy in a wizard hat talks about mixing ruby dust and newt's blood to power a teleportation spell.

But I've run into a gray area. When a story is set in the past, common knowledge of the time turns out to be quite incorrect. Combine this with the fact that it's a fantasy setting with subtle magic, it could lead to confusion.

The following is a perfectly reasonable bit of dialogue for this world. The adult, Katell, is a "Beastmaster" whose connection with animals is stronger than empathy but weaker than Aquaman Aquatic Telepathy.

CHILD
Did you ever talk to a shark?

KATELL
Well, a shark is a fish, and
most fish are pretty dumb.
The only fish that understand
anything are dolphins.

It fits into the worldbuilding, but I wouldn't want a reader to think I, the writer, am an idiot. The story is set in the Upper Paleolithic. You can't swing a dead cat without hitting some pre-scientific fact they know is true and we know is false.

As long as you establish that there is magic realism in your world early on, like in the first ten minutes, then you'll be fine. If you try to play it straight and drop something like that later on the audience, you risk alienating them.

The movie Wanted defies physics. It puts that front and center with a cold open, and that cold open is there because what follows is a guy going through his crappy ordinary life for like 20 minutes. Because of the cold open, we're waiting for him to get to that physics-bending stuff instead of being put off by the rules of the world suddenly changing.

The magic realism isn't the problem, it's calling a dolphin a fish with a straight face.

If the time period is clear, and you keep everything correct to that time period, then any reader who has an issue with it is the idiot, not the writer.

What does it matter anyway, are you worried that the ignorance of readers will hold the script back in comps and stuff?

You could also run into the reverse, as well. Someone picking you up on not being historically accurate by using today's knowledge

I'd avoid it, personally, by simply saying 'sea creature' rather than fish when talking about the dolphin.

It's in dialogue. It's a character misconception. It doesn't paint you as an idiot, just the character as ignorant.

It's hard to say without the context of the greater work. Nothing wrong with a character being ignorant. But I'd have to wonder if the writer didn't make a mistake if this were the single ignorant thing the character ever said (or if the dolphin thing didn't come up again at some point). Saying "sea creature" instead could work, but I don't think it'd flow the same way and could come off as the writer bending over backwards trying to work around the fish thing... or maybe no one would notice.

Again, hard to say taken on its own. This would be my knee-jerk reaction though. I think it's pretty sound for now.

Rather than changing the terminology, you could always have them say something like "Dolphins, on the other hand..." and go from there.

EDIT: Glossed over the fact that this is set in the past. Think you might be all good depending on your "when." Alas, I'm always prepped to overthink.

There's no advantage to leaving something like that it in. But I'd be more concerned with the rules you're establishing and how to introduce them organically.

If she's a beastmaster who can talk to creatures why limit yourself to dolphins? It's your world, you can do what you want, but you're limiting yourself to one creature in the entire ocean she can talk to.

And Katell kind of sounds like she's listing out rules in rote fashion. It's good to set the rules, just try to hide it a bit more. Maybe something like...

CHILD
Did you ever talk to a shark?

KATELL
Not if I could help it. They're a terrible
bore. Dolphins on the other hand can
be quite clever.

-----------------------------------------

You could also use the correct answer as a way to begin establishing the roles and personality of your characters. If Katell is a teacher/mentor...

CHILD
What fish have you spoken to? Sharks...
even dolphins?

KATELL
Sharks can be a bit testy. And dolphins
are mammals not fish. We'll have to teach
you the difference before you do any talking
to anyone.

------------------------------------------------

Or if she's the ignorant/haughty Aunt...

KATELL
I've spoken to all kinds of fish. Sharks have a nice
bite to them. Dolphins are a bunch of know-it-alls.

CHILD
But dolphins are mammals.

KATELL
What a silly idea, dolphins are most certainly fish.
You don't see them walking on land, do you?

Child looks unconvinced.

To give this a little more context, it's much later in the Kord story from which I previously posted the first few pages. A Beastmaster's day-to-day is mostly dogs, which at this point in real history humans had only JUST started domesticating.

Edit: Okay, the above wasn't actually a useful amount of context. Within the first ten minutes of the story, it's established that these primitive humans share the world with primitive versions of dwarves, elves, ogres, and a few other beasties. While I slow-walk the introduction of magic, its presence shouldn't surprise anyone.

Humans have just started specialization of labor, consistent with our own history about 40,000 years ago, which roughly coincided with the domestication of animals. The ones who handle animals are Beastmasters, the ones who go out in canoes are Mariners, the ones who make tools are Knappers, and so on.

Each craft has people who use magic and people who don't. The magic-y Beastmaster will have this strong link with a few animals; the non-magic-y one will be out there training hundreds of sheep.

This particular Beastmaster is of the magic-using variety and will eventually connect the dots across different crafts to discover magic as a craft of its own. History records her as being the first Wizard. But at the time of this scene, she's merely a Beastmaster worried that not enough kids join the Beastmaster craft to keep it viable. So she's watching a bunch of kids and trying to impress them.

A better attempt might be:

CHILD
Did you ever talk to a shark?

KATELL
All a shark ever thinks about is
"Food! Food! Food! Food!" But a
dolphin likes to play.

A dolphin fifty yards offshore leaps from the water and flips before diving again.

Thanks for your advice everyone. Looks like I found a way to write around this gray area, but I won't be as worried about others that show up.

I'm watching your movie.  I understand it's a supernatural fantasy set in the past.  A character refers to a dolphin as a fish.  I think hmm...did people know back when this movie is set that dolphins were mammals, not fish? Or is this a matter of the screenwriter not knowing a pretty common fact?  Fantasy or not, am I supposed to understand that this intelligent character got their facts wrong?  Is it the character who's not as intelligent as we've been lead to believe, or is the screenwriter's idea of intelligent not as intelligent as they think?  Or is it a matter of the character -- oops, in the midst of all these questions, I've stopped paying attention to the movie.

A bit exaggerated, maybe, but still.  If you have any question at all about your script, the reader likely will, too.   I'd avoid the potential distraction by either having the character get their facts straight, or just leaving the comment out altogether.  

If vyoiu wanted to show the kid doesn’t know - have the expert say back - A dolphin is not a fish. And the kid give a wise answer back like “tell them that or “they live in the ocean don’t they?”

Humans had been hunting whales for over three thousand years before figuring out that they weren't fish. The word whale evolved from a term for "big fish." On the assumption that common knowledge about dolphins was similar, even an animal expert from 40,000 BC would firmly believe that a dolphin was a fish.

Since I don't want to dwell on this long enough to explain the mistake, avoiding the dolphins-as-fish statement is definitely the way to go here.

Quoted from MarkItZero There's no advantage to leaving something like that it in. But I'd be more concerned with the rules you're establishing and how to introduce them organically. If she's a beastmaster who can talk to creatures why limit yourself to dolphins? It's your world, you can do what you want, but you're limiting yourself to one creature in the entire ocean she can talk to. And Katell kind of sounds like she's listing out rules in rote fashion. It's good to set the rules, just try to hide it a bit more. Maybe something like... CHILD Did you ever talk to a shark? KATELL Not if I could help it. They're a terrible bore. Dolphins on the other hand can be quite clever. ----------------------------------------- You could also use the correct answer as a way to begin establishing the roles and personality of your characters. If Katell is a teacher/mentor... CHILD What fish have you spoken to? Sharks... even dolphins? KATELL Sharks can be a bit testy. And dolphins are mammals not fish. We'll have to teach you the difference before you do any talking to anyone. ------------------------------------------------ Or if she's the ignorant/haughty Aunt... KATELL I've spoken to all kinds of fish. Sharks have a nice bite to them. Dolphins are a bunch of know-it-alls. CHILD But dolphins are mammals. KATELL What a silly idea, dolphins are most certainly fish. You don't see them walking on land, do you? Child looks unconvinced.

Great advice.

Like I said, these folks would logically have a zillion “certain truths” that we know are wrong. They know that the world is flat, that metal boats would sink, that thoughts come from the heart and emotions from the liver, that the Moon fights with the Sun every decade or so with the Sun always prevailing after a few minutes, etc.

I’m perfectly fine with using any of those if there’s an organic way to explain the misconception... it’s just that the fishiness of dolphins would have been left unchallenged.