Identify the incorrect phrase: The mail carrier must wear their uniform at all times.

The phrase "their uniform" is considered incorrect in this context due to the traditional rules of pronoun agreement in standard English grammar. In this sentence, the singular noun "mail carrier" indicates a single individual, and traditionally, singular nouns are matched with singular pronouns. The pronoun "their" is plural, which can lead to confusion when paired with "mail carrier."

A more grammatically appropriate phrase would use a singular pronoun, such as "his or her uniform," to maintain agreement in number. This adjustment clarifies that the uniform belongs to one specific individual, aligning with the singular subject.

While using "their" as a singular, gender-neutral pronoun is becoming increasingly accepted in modern usage to promote inclusivity, strict adherence to traditional grammar rules would indicate that "his or her" is preferred in this situation. Thus, the incorrect phrase is identified as "their uniform."