The suspension of disbelief is something every writer hopes to instill in his or her readers. Writers are liars by trade. They invent people, places and situations that never existed (and in some cases, never could exist) and strive to make total strangers care about them. Fortunately, there are plenty of people out there who are looking to do just that. Still, it's possible to fall flat. And even the most successful writers won't please everyone on every front.
People who write stories set in the "real world" have a particularly difficult task, because there are so many little details to keep straight. And some authors (evidently) don't even bother. My mom recently told me about a mystery she stopped reading because the protagonist had a close encounter with a shark that was so completely un-sharklike (it displayed almost human intelligence and seemed to be able to breathe with its head out of the water), that she started giggling.
I tend to get annoyed by books or movies that portray science inaccurately or that perpetuate myths or stereotypes that I despair of (like that stupid idea that we only use 10% of our brain). Some people would say I'm not picky about these issues. But my military history buff friends are far more likely than I am to get knocked out of disbelief by a scene that incorrectly portrays military ranks or organization, or has the wrong type of planes appearing in the Battle of Midway.
I write fantasy, and so in some ways my job is easier than an author who writes realistic novels set in the so-called real world. Fantasy readers are coming to the table with an expectation that at least some of the things in the books they read will be, well, fantastic. There's usually a huge leap of faith you have to make when you pick up a fantasy novel. We all "know" magic doesn't exist and there are no such thing as werewolves or dragons or parallel worlds that exist beneath the streets of London, but we're prepared to believe for a while--so long as the writer creates an interesting premise and compelling characters.
Still, a fantasy writer doesn't have free reign. When I create a setting for my story, I can decide what type of world it's going to be and how its rules will differ from the so-called real world. If I want to take a page out of CS Lewis's book and make my world flat (and only a few thousand years old), that gives me more latitude if I want to do something like, say, make the moon and sun go around the world and turn stars into sentient humanoid beings. But if I create a fantasy world that is essentially Earthlike, except for the presence of magic, I have to be a bit more careful when I describe the physical workings of that world.
I ran into a suspension of disbelief issue when reading one of my favorite authors. Her world is very Earthlike (one sun, one moon, evidently round) and seems to have the same basic laws of physics as we do, with a few exceptions made for magic. But for some reason, the moon always rises at dusk and sets at dawn, no matter what phase it is in. I can't think of a way for this to be possible. A solar eclipse is a major event in one of this writer's books, but on the morning of the eclipse the moon was setting at dawn. If the moon is setting at dawn, it's not going to be in the sky during the day to get exactly between the sun and the Earth (to cause an eclipse). This knocked me out of disbelief.
But I didn't stop reading, and I still enjoyed the book. Except for the moon thing, her writing is very well informed and I love her plots, world building and characters. She has, in essence, earned my trust as a reader, so I am able to forgive that small slip (one that many readers probably don't even notice).
The fact is, no matter what a writer does and no matter how carefully he or she researches things, there will be some mistakes or omissions. I notice little unlikely things in books all the time (and undoubtedly miss countless others). George RR Martin had a character tossing hay bales around in Game of Thrones, for instance, even though it's unlikely something akin to the 1930's era pickup baler would exist in his medieval-ish world. It was a very minor thing, however, and I still enjoyed his books. I don't think the plot ever hinged on the presence or absence of hay bales.
And sometimes an author has to fudge something to make a story work. I know that something the size of a dragon can't possibly fly (without magic), but I was able to set that knowledge aside and enjoy Anne McCaffrey's Pern books (which are actually science fiction and not fantasy). At some point, a writer has to set things up as best he or she can and tell the story.
Ultimately, readers all have certain tropes or situations they are more willing to accept than others. For example, there are all kinds of "realistic" reasons why women in pre-industrial societies did not (generally) pursue lives of adventure. But I like to read fantasy novels set in sword and sorcery style settings that are "unrealistically" egalitarian. This is an example of a trope (swashbuckling female adventurers/heroes) that has become pretty commonplace in modern fantasy and is acceptable to a large number of modern readers. But 50 years ago, novels with a sexually integrated military (even in futuristic settings) and sword-wielding female heroes that were portrayed as everyday, normal occurrences, probably would have knocked many more readers out of disbelief than they do today.
Of course, even in a modern novel, there are ways of portraying this integration of male and female roles that will be perceived as more or less realistic to some readers. For instance, an understanding of the frictions and issues that can arise in heavily co-ed settings, even in our relatively enlightened times, can make a scenario seem more realistic.
In the end, readers will be inclined to accept or reject certain things, based on their past reading habits and personal knowledge or inclinations. But the fact that a story takes place in a fantasy world does not abrogate a writer from doing some research and creating a set of internally consistent rules. If the writer does this, readers are much more likely to be forgiving when he or she does fudge something--whether it's by omission or design.