In Downum v. United States, the U.S. Supreme Court ruled in favor of petitioner Jack Downum, who was charged with robbery and had his trial postponed after a jury had been sworn in but before any evidence was presented because a key prosecution witness was absent. The court later attempted to retry him with a different jury. Downum argued that this violated his Fifth Amendment right against double jeopardy - being tried twice for the same crime. The Supreme Court agreed, holding that once a defendant is placed in jeopardy by having a jury sworn in, subsequent attempts at prosecution for the same offense are prohibited under the Double Jeopardy Clause of the Fifth Amendment unless there were manifest necessity for discharging first jury or consent by defendant to discharge it without verdict.

In the dissenting opinion for Downum v. United States, Justice Clark argued that the majority's decision to overturn Downum's conviction on double jeopardy grounds was misguided and inconsistent with precedent. He contended that there had been no intention by the prosecution to harass or oppress Downum through multiple prosecutions, which is what the Double Jeopardy Clause of Fifth Amendment primarily protects against. Instead, he asserted that it was a simple case of prosecutorial unpreparedness due to an unexpected absence of key witnesses - a situation not uncommon in criminal trials and one which does not warrant dismissal under double jeopardy principles. Furthermore, he emphasized that dismissing charges in such circumstances could potentially lead to abuse where defendants might strategically cause delays hoping for dismissals based on similar technicalities.