When is it appropriate to break the rule that requires subjects joined by and to use a plural verb?

It is appropriate to break the rule requiring a plural verb for subjects joined by "and" when the subjects refer to a single entity or concept, such as in the case of a compound noun (e.g., "peanut butter and jelly is my favorite sandwich"). Additionally, if the subjects are considered collectively or represent a single idea, a singular verb may also be used. Context and meaning should guide the decision for proper subject-verb agreement.