Certificates of insurance issued by insurance agents or brokers do not confer coverage by themselves according to court case in New Jersey. As Mound Cotton reports (see here), a decision on the subject was reached relying, in part, on two decisions in New York. The court concluded that certificates of insurance issued by a broker are not binding insurance contracts.
These cases illustrate that a certificate holder should not rely on a certificate of insurance alone as evidence of insurance coverage.
It appears that coverage questions involving additional insureds arise with some frequency because certificates are sometimes issued noting incorrect additional insureds.