As noted in the main text, "Willing Suspension of Disbelief" (WSD) is the idea that the audience of a story chooses to put aside their knowledge that the story is fiction and invest a certain amount of temporary "belief" in its events—however unlikely they might be in the real world—in order to enjoy it as much as possible.
It is a reciprocal arrangement, though—implicit in the audience's willingness to engage the story on its own terms is the expectation that the creator of the story will "play fair" with them—that once he establishes the "ground rules" of the story (including things like style, tone, and genre, as well as setting and flavor) he will not change them without warning and without a very good reason, nor will he do anything to remind them that they are in a story that does not actually serve the purposes of the story.
Note also that WSD is not a bargain struck at the start of a story and from then treated as permanent. It is a constant state of negotiation between the creator and the audience. Every new element the creator adds to the story is evaluated by the audience in terms of everything that has come before, whether they do so consciously or not. If it looks like the new element conflicts with or contradicts the "ground rules" of the story as they have evolved up to this point, the audience will react negatively.
Think of it this way: the beginning of a story is like a half-built jigsaw puzzle. As the story goes on, the writer hands new pieces to the audience, who test each one for how well if fits into the puzzle. If the creator is doing his job right, each piece he gives the audience will fit in right away, or at least look like it will fit somewhere soon. However, if it's too hard to fit a piece in, the audience may set it aside and ignore it. Too many of those, and they may give up on the puzzle. And if it looks like the piece is from another puzzle entirely, they may get frustrated or even angry, then give up on the puzzle. They may even throw the puzzle to the ground, convinced that the creator is jerking them around instead of helping them to enjoy it.
Here's a more concrete, if somewhat extreme, example. Imagine you're watching The Bourne Identity, or any other similar spy/techno thriller movie. It's tense, it's exciting, it's filled with classic spy gadgets and gambits and head-games, with car chases and briefcase bombs. Then when you get to the climax, the big bad pulls out a wand and stuns the hero with a spell. Up to this point there's been no suggestion that this is a world with magic, no subtle hints or clues that there is anything going on in the movie besides high-tech cloak-and-dagger skullduggery. You say, "...the hell?"
The bad guy with the wand is the puzzle piece that does not fit, no matter how you try to jam it in. It breaks all the rules the movie has convinced you to accept in order to enjoy the story. You've just had your willing suspension of disbelief violated.
The usual response of a reader to a violation of WSD depends on the degree of the violation. Minor ones may not draw much attention to themselves but still evoke a feeling of "wrongness" in the reader. If there aren't many of these, the reader may keep reading, but finish the story with the feeling that is "bad" without being able to precisely put a finger on what made it so. Mid-level violations will be obvious to the reader, who may or may not forgive them while reading (but will likely say "that's stupid!" regardless); when done he will have no doubt that the story was disappointing. If there are large violations, or lots of smaller ones, the reader may abandon the story completely, and tell people not to read it.
Note that "playing fair" with the audience doesn't mean a creator has to lay everything out for them right from the start. He just has to give them enough to build a base that they understand and agree to, consciously or not, on which he can then add more as the story goes along.
And a creator deson't have to make the implications of a story element immediately apparent—it's a good trick of the trade to get the reader to accept an apparently innocuous detail without making a fuss about it, and then when they least expect it, drop an unexpected plot point on them that is an eminently logical consequence of that detail.
(Text adapted from A Fanfic Writer's Guide To Writing by Robert M. Schroeck, with the permission of the author.)