The sentences with incorrect subject-verb agreement are:
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Either the serving platter or the bowl are in the bottom cabinet next to the dishwasher. (The verb "are" should be "is" because "either...or" takes a singular verb when referring to singular nouns.)
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The people who attended the fair was there because they were interested in purchasing artwork. (The verb "was" should be "were" to agree with the plural subject "the people.")