The soothsayer tries to warn Caesar about the ides of March. "Caesar!... Beware the ides of March." (Act I, Scene ii, Ln 12). He warns Caesar about March 15th but does not tell him why. This leads Caesar to believe he is crazy.
Act IV, Scene i
Caesar goes to work instead of listening to his wife. "How foolish do your feels seem now, Calpurnia!" (Act II, Scene ii, Ln 100). They did not find a heart in a slaughtered goat which made Caesars wife scared. Caesar was convinced to go to work anyway.
Act V, Scene iii
Caesar tries to tell the soothsayer he is wrong in worrying in the ides of March. The soothsayer reminds him the day isn't over. "Ay, Caesar, but not gone." (Act III, Scene i, Ln 2). Caesar thinks he has proven the soothsayer wrong.
Lepidus is asked if he is okay with the conspirators killing his brother. Lepidus does agree almost without thought. "Your brother too must die; consent you, Lepidus? I do consent-" (Act IV, Scene i, Ln 2-3). Lepidus is trying to prove he will do anything to be on their side.
Cassius kills himself because he thinks he just sent his friend to his death. "O, coward that I am, to live so long, To see my best friend ta'en before my face." (Act V, Scene iii, Ln 130). He kills himself from being dramatic and early to pull the trigger. He never killed his friend but he didn't live long enough to see it.
Keep your friends close but your enemies closer. Antony convinced Brutus that he was no threat and let his guard down. "Friends I am with you all, and love you all." (Act III, Scene iii, Ln 220). Since Brutus let his guard down on Antony there was a war that Brutus eventually lost.