These are terms I learned in a linguistics class in grad school. If you're not familiar, here are the quick-and-dirty definitions.
Prescriptive grammar is grammar according to the super-official grammar books.
Descriptive grammar is how people actually talk.
Of course, language is always evolving, and often the changes come because something in the realm of descriptive grammar becomes so common and pervasive, it overwrites the prior rule in the prescriptive grammar books.
In certain arenas, it's appropriate to follow prescriptive grammar rules to the letter. When writing fiction, it's not so clear-cut. There's also voice to consider. Dialogue in particular gets a little more leeway when it comes to grammar.
Once in a while, though, something comes along that can't be explained away by voice, and yet I can't bring myself to write it the "proper" way because my gut says we're on the verge of overwriting the rule. (Or at the least, my gut says people who talk that way in real life are a critically endangered species.)
For example, in my current project, I have a character say, "It is her." (The sense is, "She is the one we're looking for.")
Gerty Grammarian says it should be, "It is she." In the particular situation, it makes sense that the character would be fairly educated and would probably speak in a proper manner.
But I can't bring myself to write it that way. It just feels too wrong.
In a situation later in the story, a similar line came up, and in that case I did change it. I wanted that particular character to be over-the-top formal, so it made sense to me. It felt right.
How about you? Do you have any little gems of grammar that you know are "correct" one way, but you just can't bring yourself to write it that way?