2. The problem, caused by accounting errors, costs the company hundreds of thousands of dollars.
(The subject is "problem," not "errors.")
3. Mr. Hudon, not the members of his department, is responsible.
(The subject is "Mr. Hudon," not the plural word that comes between the subject and the verb.)
4. Carelessness along with poor supervision is responsible for these problems.
("Along with poor supervision" is not part of the subject. The subject is the singular word "carelessness.")
5. One of the supervisors is going to be fired.
(The subject is "one." not "supervisors.")
6. A series of such mistakes points to the need for reform.
(A series is a singular thing.)
7. No criteria have been established to evaluate our losses.
("Criteria" is plural. The singular form is "criterion.")
8. A number of the associates is/are going to see a pay cut.
("A number" is plural in meaning here.)