In Shakespeare's play Julius Caesar, Cassius makes a devastating mistake in Act V. The outcome of this devastating mistake will be death.
At this point in the play, Caesar has been assassinated. In the wake of his death, war has broken out. Just before Cassius makes his fatal mistake, he is standing with Titinius, discussing the battle happening in front of them.
A messenger arrives to say that the enemy's troops are getting closer. The messenger advises Cassius to run and escape. Cassius, for his part, refuses. Then, Cassius sees troops in the distance, but he cannot tell whether the troops belong to him and his allies or to his enemy, Antony.
At this point, Cassius gives Titinius his horse. He tells Titinius to find out the identity of the troops. Titinius agrees and rides off. Cassius then instructs the messenger to watch Titinius and report to him how he is doing. Because he is watching from a distance, the messenger believes he is seeing Titinius captured and killed. As a result, the messenger tells Cassius that Titinius is dead.
Cassius hears shouting below and believes the messenger's report. The outcome of this mistake is that Cassius believes all is lost and kills himself.
In reality, Titinius is not dead. They were not enemy soldiers he encountered below -- they were allies. The cries and shouts were shouts of joy, not war. Cassius took his own life for no reason -- a devastating (and fatal) mistake.