And topping the list of easy-to-make verb-agreement errors are what are called relative-pronoun-antecedent-agreement errors. Take, for example, this sentence that came up in my editing this week.

Rules for Pronoun-Antecedent Agreement. Nouns joined by "and" are treated as plural. Ex: The player and the coach decided their score. When nouns are joined by "or," the pronoun agrees with the ...