The Tennessee Valley Authority (TVA) started building of the Tellico Dam in 1967. During construction, an endangered fish species was found upstream. In 1973, Congress passed the Endangered Species Act (after construction of the dam had commenced. The reservoir created by the dam was thought to be harmful to the species of fish in question; action was taken to halt construction (which by then was nearly complete).
The Supreme Court considered whether the ESA required a court to enjoin the operation of the Tellico Dam (which had been authorized prior to 1973) when the Secretary of the Interior had determined that operation of the dam would eradicate an endangered species. TVA v. Hill, 437 U.S. 153 (1978).
Section 10 of the ESA (codified as 16 USC § 1539) provided a number of "hardship exemptions" to the ESA. However, none of these "hardship exemptions" applied to the Tellico Dam project. Furthermore, Congress had explicitly provided appropriations for the Tellico Dam even after the ESA was passed in 1973 (an implied repeal argument that was ultimately unsuccessful, but is outside the scope of this discussion).
Relying in part on expressio unius est exclusio alterius, the Court concluded that the Section 10 "hardship cases" were the express intent of Congress; and the exclusion of other situations meant Congress did not intend any further exemptions. If Congress had intended for Tellico Dam to be completed, the Endangered Species Act be damned (so to speak), they could have said so expressly. Indeed, Congress later explicitly authorized completion of the dam despite the ESA's provisions. This doesn't make what the Court did wrong; in fact, it validates it.