This Week's Most Common Error

I listen to the radio a lot when I drive. One of the most common errors I picked this week was not only from radio talk shows but also TV prgrams in which news anchors get into semi-formal discussions on issues with each other: "If a person...they shouldn't...." Pronoun number agreement. I think this is what most people say. I did too because I didn't realize there was such a thing as number agreement in grammar but mostly because that's how everyone spoke. Besides, it's so much easier to pick out the mistakes in listening to others than in our own.