Promises are made to be broken. Hmm. Nope. Scratch that. Promises are made to be broken well or not at all. Either you have to keep the promises you make to your readers, or you have to break them in a way that doesn’t really break them.
Let’s just say that breaking them is the exception, not the rule.
But if you want to break one of your main promises (like that the rules of the world will be followed), then there should be a reasonable (*cough* foreshadowed *cough*) explanation for why the rule wasn’t really broken. The four main options are 1. character ignorance, 2. a change in the situation/experience, 3. an exception that requires extreme effort (which usually also involves the first option), and 4. the unreliable narrator.
1. Character Ignorance
The character in question is the main character or any character whose point-of-view you write from. The gist of character ignorance is that the rule was always broken because the original understanding was wrong (a mistaken assumption, missing information, a lie, a secret, etc.) – that character simply didn’t know that until now.
For instance, if the broken rule is that Tracy doesn’t like chocolate, and here she is eating chocolate, then maybe the main character misunderstood, and Tracy just doesn’t like milk chocolate (but loves dark chocolate). Or maybe Tracy lied earlier when she said she didn’t like chocolate because she didn’t want to accept the melty chocolate from your weird coworker’s pocket. Or because of embarrassment, Tracy was simply trying to hide her chocolate addiction (Shh! It’s a secret! …well, it was.).
In other words, there’s a specific reason that the main character’s impression of that rule was faulty, and the reader learns that at the same time as that character.
2. A Change in Situation/Experience
As Badger said in Firefly, “Crime and politics, little girl. Situation is always… fluid.” Characters react differently in different situations. If a character is faced with the same choice in a different situation, the character’s answer can change without making the reader feel cheated. A standard trope of this is the scientist who refuses to create the doomsday whatever. Then, the villain kidnaps the spouse/child, and the answer reverses. Different situation + different answer = no broken rule.
The other part that can change is the character. Characters have learning curves, too. When they experience something new, they learn from it. Or they might react to it emotionally. The resulting changes can change their reactions. For example, if a naive character agreed to something that ended poorly at the beginning of the book and is offered another “wonderful opportunity” later on, a different reaction is very believable. The big rule of this particular technique, however, is that the reader needs to see that change. Then, rule isn’t broken because the rule changed, and the reader knows that.
3. The Exception That Requires Extreme Effort
I say that the exception requires extreme effort because if it’s easy, it’s not an exception. It’s a variation on the rule. It’s a little like the U.S. Congress’ ability to overturn a Presidential Veto. The rule’s been in place for forever, so it’s not like someone’s making it up just to make a plot point work. On the other hand, a 2/3 majority of the House and Senate isn’t all that easy. It’s possible, but it’s not going to be your go-to answer for everything.
Granted, in many stories (fantasy, especially), the exception is a closely held secret by the elders/higher ups/secret society/lost lore, so the characters have to first learn about it and then go through the hoops to get it. But watch out for making it too forced, especially when the rules were well-established through several books or films already (“Oh, yes, if a pig comes by Castle Dracula on a Tuesday, playing a banjo… kind of crowbarred plot move…” — Eddie Izzard).
This is also a gimmick you should really only use once per book, possibly once per series.
4. The Unreliable Narrator
The final excuse is, “The narrator lied.” Well, maybe, the narrator believed it at the time. Maybe, the narrator is a chronic liar, and the whole narrative was a lie. Both are possible; however, the breadcrumbs that validate the narrator’s unreliable nature have to be there when the reader looks back, or this is cheating (and it’s gonna make ’em mad). It’s hard work to do this well. So unless you really want to write an unreliable narrator, it’s easier to keep the promises you make rather than hide a bunch of false promises in the story.
Actually, if you’ve been paying attention, you’ve probably noticed that all these exceptions require at least the same amount of work as keeping your promises. If not more. That means that they’re not easy cheats to get out of hard work. Nope. What they are is useful techniques if you want to set up a false promise and mislead the reader. It won’t save you any effort, but it can let you break your promises without breaking your promises.