Source: Public Domain Pictures
I have come to realize that in that in writing we make a lot rules but those rules have many exceptions. So, my working theory is that in writing there are no concrete rules. No rules without exceptions.
Grammar
So, it might spring to mind that grammar rules should always be followed. However, take for example if the author is writing dialogue that shows an accent or dialogue they might intentionally spell something wrong.
Source: Wikimedia Commons
Correct by grammar rules:
“Whom do you think ate the last snickerdoodle?”
Exception:
“Who do you think ate the last snickerdoodle?”
When quoting a character in a book you would probably use the latter phrase because this is how most people would say the sentence. The grammatically correct version would stick out as an unrealistic unless the character you are quoting has an affinity for correct grammar.
Spelling
You might immediately reply “what about spelling?” 
Well, in the English language it seems that there are sometimes more exceptions than rules. Take for example the rule i before e except after c. Well, not only is there an exception within the rule but there also exceptions to that rule. Take for example the incomplete list below. All of those words break that rule (here is the complete list if you are interested)
Additionally, there is disagreement between English-speaking countries on spelling.Take for example the British spelling of colour versus the American spelling of color.
Source: Wikimedia Commons
Or cancelation versus cancelation. In different parts of the English speaking world things are spelled differently so clearly spellings have exceptions depending on your location. Here is a very long list of words we spell differently from the Brits if you want to see just how many differences there are.
Format
Well, format then perhaps? Different genres have different punctuation expectations, different spacing expectations, and different vocabulary expectations. Poetry often lacks punctuation and uses as much of or as little of the space as it desires. In contrast prose uses extensive punctuation and has many rules about the spacing. Take for example an academic essay in comparison with a shape poem (see image to right). I can’t distinguish any clear rules that those have in common.
Look to Poetry for an Example
Often to find the exception to a rule just look to poetry. You can CAPITALIZE words, italicize words , and bold words. And it is all within your artistic liberty to do so. You can use no punctuation or a ton of it. E.E. Cummings is the classic example of a poet that experimented with capitalization and punctuation (here are some of his poems).
Context
Whether the exception is used well or not depends on the context of the writing. If I use all capitals in an email to my professor, she will regard it as rude. So, yeah the rules in writing are there for a reason, but it does not mean that they are to be worshipped as the one and only way to write.
While much of the time these writing rules serve to make our communication more standard, efficient, functional, and appropriate to context, that doesn’t mean they should be followed without regard for the context of the situation.