Please give the meaning of these lines spoken by Messala in Act V Scene iii of Julius Caesar:
Mistrust of good successe hath done this deed.
O hatefull Error, Melancholies Childe:
Why do'st thou shew to the apt thoughts of men
The things that are not? O Error soone conceyu'd,
Thou neuer com'st vnto a happy byrth,
But kil'st the Mother that engendred thee.
There's an interesting analysis of "error" in the text that goes beyond the action of the play. Shakespeare conveys what went wrong with how Cassius perceived the intelligence of how the battle was unfolding. Before his suicide, Cassius sends his servant Pindarus to observe Titinius; Pindarus reports what he sees -- that cavalry have crowded around Titinius, and pulled him from his horse. What Cassius interprets is that his friend has been overrun by the opposing army and has been killed. In reality, it was his own side celebrating Titinius' arrival, having survived combat in his section of the battle. Several passages later, Titinius addresses the dead Cassius, saying "Thou hast misconstrued everything."
And indeed he has. When Cassius bids Pindarus to describe the battle, he says:
Come downe, behold no more:
O Coward that I am, to liue so long,
To see my best Friend tane before my face
So Cassius errs in interpreting the events Pindarus reports, thinks his friend is dead, and so commits suicide. What Messala then comments on in general is how melancholy thoughts give rise to errors in perception. In seeing the worst and interpreting it to be true, tragedy results, as one always acts on what one perceives to be true. "Apt thoughts of men" is a reference to perception and thinking, "Things that are not" is a reference to falsehoods. Shakespeare makes a metaphor of offspring, conception, birth, and death -- What used to be called "Melancholia," or what we would call depression or negative thinking, creates negative thoughts ("errors") that create negative perceptions, that acted upon, cause destruction.
In this scene, the armies of Brutus and Cassius have been fighting those of Antony and Octavius. In the confusion, Cassius believed everything had gone wrong and he had Pindarus kill him (sort of like commiting suicide). This is the context for the lines you give. They mean:
He killed himself because he didn't realize what had really happened. Why do people always get fooled into believing things that aren't true? His sadness made him make mistakes and then it killed him.
Or words to that effect. Basically he's mourning the fact that Cassius has died because he didn't understand what had really happened.