The error made in the sentence "One of the boys left their jacket in the locker room" is a very common one. People tend to focus on the plural word, "boys" in this case, and use the possessive according to it. But the fact is that we are not talking about the "boys", we are talking about a boy, one of them. Therefore, the possessive pronoun should not be a plural one, but a singular one. The proper sentence would be:
- One of the boys left his jacket in the locker room.
