Let's talk about something. Let's talk about the "singular" they. That's when a writer or a speaker — a he or a she — is discussing someone who might be either a he or a she (it's unknown, or the writer doesn't intend to make a subject or object gender-specific and instead hopes to convey a universality of personhood). So instead of writing, say, he or she did x or y, the writer uses they. It's everywhere, proliferating like fruit flies 'round a bowl of rotting bananas, bad writing surrounding bad writing. Some examples:
"If someone is concerned about their mental health, they should seek professional help."
"If a person decides they like dubstep, that's really their prerogative."
"What do you say to a coworker when their attitude is just terrible?"
These all make me cringe, but it's a usage that has a fair number of supporters. In a post on The Economist's Grammar blog, R.L.G. examines the matter further, as inspired by grad student and blogger Freddie deBoer, who thinks we need to stop fighting the use of their as a singular pronoun. deBoer writes,
Using "their" for singular antecedents is one that I think people need to just give up on. As I've argued, it only occurs in a very limited set of circumstances, and those circumstances [are very] unlikely to produce confusion about what is meant. We all know what is intended in such a statement, to the point that most of us don't even notice it in spoken conversation. And as we lack a satisfying alternative, the usage is likely to persist. That's not to say that you shouldn't understand what the "rule" is, if only to be able to satisfy those gatekeepers that police it. (Don't use it in your resume, don't use it in your grade school application.) But this is an example of a gate that's not worth defending anymore.
R.L.G. takes it from there, explaining that they was used as a singular pronoun for centuries without anyone complaining (and by many notable crafters of language, including Chaucer, Shakespeare, and Swift). He adds that not only is the usage "very unlikely" to produce confusion, but also, it's "nearly literally impossible for singular they to be confusing in an actual conversation or in a longer piece of writing." Further, "that still doesn't prove singular they ungrammatical," he writes, concluding, along with deBoer and Bryan Garner, that the singular they is "the most convenient solution" to all of our pronoun trouble. A caveat or suggestion: "Use singular they in relaxed prose, when you know you're in the company of those who get this right, or if you don't mind annoying a determined and vocal minority."