Identify the error in the following sentence: (a) The unintentional mistakes (b) made by the clerical staff (c) was overlooked by the officer. (d) No error.

The error is in part (c). The subject of the sentence is 'mistakes' (plural), so the verb must agree in number. The singular verb 'was' is incorrect; it should be 'were overlooked by the officer'. This is a subject-verb agreement error where the plural noun 'mistakes' requires a plural verb. Parts (a) and (b) are grammatically correct.