When the rule outlives common usage

"Each person needs to know what they want." "He had a real good time." "The cat licked it's paw."

I encounter these sentences in my editing work all the time and I dutifully correct them because they are all grammatically or mechanically incorrect. Here are the corrected versions.

Each person needs to know what he or she wants (singular with singular).
He had a really good time (adverbial form modifying an adjective).
The cats licked its paw (the possessive of it has no apostrophe).

But we hear and see the incorrect versions all the time, so often in fact that we absorb them right into our speaking and writing. Usage is beginning to outweigh the rules.

American English has what is called a descriptive grammar, where the grammar rule describes common usage. Other languages, like French, have a prescriptive grammar, where the rules are decided by an august body of writers and grammarians.

Those of us in the business of helping writers adhere to current rules often have to give such sentences as those above double consideration. If it is a doctoral dissertation I'm editing, all three situations would get corrected. If it's fiction and these sentences occur as dialog, I'd correct only the last one (punctuation is still correct within dialog) but I'd leave the two usages alone as part of the natural speech pattern. However, if it's fiction and not dialog and not told in first person, I'd correct the apostrophe and query the author as to what he or she means to convey by the casual usage in the other two.

Things are changing in the language all the time and we editors try to stay attentive to what's happening.