Sometimes an alleged grammatical "error" is logical not only in the sense of "rational," but in the sense of respecting distinctions made by the logician. Consider this alleged barbarism: Everyone returned to their seats.
If anyone calls, tell them I can't come to the phone.
No one should have to sell their home to pay for medical care.
The mavens [prescriptive grammarians] explain: [everyone] means [every one], a singular subject, which may not serve as the antecedent of a plural pronoun like [them] later in the sentence. "Everyone returned to [his] seat," they insist. "If anyone calls, tell [him] I can't come to the phone."
If you were the target of these lessons, you might be getting a bit uncomfortable. [Everyone returned to his seat] makes it sound like Bruce Springsteen was discovered during intermission to be in the audience, and everyone rushed back and converged on his seat to await an autograph. If there is a good chance that a caller may be female, it is odd to ask one's roommate to tell [him] anything (even if you are not among the people who get upset about "sexist language").
Such feelings of disquiet -- a red flag to any serious linguist -- are well-founded. The logical point that everyone but the language mavens intuitively grasps is that [everyone] and [they] are not an antecedent and a pronoun referring to the same person in the world, which would force them to agree in number. They are a "quantifier" and a "bound variable," a different logical relationship.
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