grammatical grey area

anyone who has tried to learn english knows that the language is notoriously weak when it comes to its adherence to its own rules. add to this the fact that there are many flavours of english around the world, and it all adds up to notable 'grey areas' in the definition of correct grammar. this post lists some examples where opinion and style often determine what is 'correct.'

the oxford comma
when you list a number of items, it can be correct to include of exclude the comma before the 'and.' this final comma is known as the oxford comma (a.k.a. serial comma or harvard comma).
for example:
I like water polo, snowboarding and grappling. (no comma)
I like water polo, snowboarding, and grappling. (with the oxford comma)

sometimes the comma is added or removed for clarity rather than style.
for example:
her favourite cartoons are the powerpuff girls, samurai jack and ren and stimpy. (ambiguous)
her favourite cartoons are the powerpuff girls, samurai jack, and ren and stimpy. (clear)
the second sentence is less ambiguous since it's clear that 'ren and stimpy' is one item in the list. in the version without the oxford comma, it's possible that 'samurai jack and ren' could be a cartoon.

possessive apostrophe
most people omit the final 's' when they write the possessive form of a name that ends with an 's.' however, according to Fowler's Modern English Usage, the final 's' should be added when the possessive form of the name is pronounced differently than the non-possessive.
for example:
that is Charles' bike. (common but incorrect)
that is Charles's bike. (correct)
exceptions to this rule include "names from the ancient world" (e.g. Achilles'), names that end with an "iz" (e.g. Moses') sound and Jesus' (source: Eats, Shoots and Leaves).

the split infinitive
the most commonly cited example of this error is from the famous introduction to Star Trek: "to boldly go where no man has gone before." many people argue that "to boldly go" is grammatically incorrect because 'to' should be next to 'go.' however, there is plenty of debate about the split infinitive rule.

vocabulary
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over time, the meaning of words can change. this can prompt a writer's choice between the vernacular and the traditional meaning of a word.

ambivalent
today this word is commonly used to mean that one doesn't hold an opinion about something. however, the actual meaning of the word is "coexistence in one person of opposite feelings towards same [sic] person or thing" (source: Oxford Dictionary). so what do you do? do you use the word to mean what people expect, or do you use the traditional meaning?

father vs. further
from http://www.andrew.cmu.edu/course/76-451/watts.html
"the Little, Brown Handbook ... says that "farther refers to additional distance . . . and further to additional time, amount, or other abstract matters" (p. 800). Again, this is a simple guideline, but one that Merriam-Webster does not agree with:

Farther and further have been used more or less interchangeably throughout most of their history, but currently they are showing signs of diverging. As adverbs they continue to be used interchangeably whenever spatial, temporal, or metaphorical distance is involved. But where there is no notion of distance, further is used . Further is also used as a sentence modifier , but farther is not. A polarizing process appears to be taking place in their adjective use. Farther is taking over the meaning of distance and further the meaning of addition .

Merriam-Webster clearly states that the two words are fundamentally interchangeable, with only partial specialization. The Handbook makes the case that the words are already distinct, though not in quite the manner Merriam-Webster predicts. However, ... neither guideline is an accurate reflection of usage, in the United States, Great Britain, or Australia."