Disagreement of pronoun and antecedent. Both problems seem to be invisible to many, perhaps because disagreement is common in the spoken language.
People speak loosely and tend to do the same thing when they write.
Unless one is writing dialogue, this doesn't work. Discipline is necessary for good writing. Subject-verb disagreement tends to take three forms. A subordinate clause intervenes between the subject and the verb. The only maintenance the plastic tables need are cleaning, Fuller said.