William E. Arnold Co. v. Carpenters

Holding that "[w]hen an activity is either arguably protected by 7 or arguably prohibited by 8 of the NLRA," federal courts must defer to the NLRB, but that when "the activity in question also constitutes a breach of a [CBA], the [NLRB's] authority is not exclusive and does not destroy the jurisdiction of the courts in suits under [LMRA §] 301"