Of what flaw does Antony accuse Brutus in Julius Caesar?This is the same flaw that Cassius accuses Antony.
Your question concerning Shakespeare's Julius Caesar is a bit vague, but I think you're referring to the opening of Act 5.
In the opening parlay of the scene the four commanders trade insults before they dismiss to prepare for battle. Antony accuses Brutus of having "good words" (line 31) and Cassius accuses Antony of having sweet words (lines 36-37).
Antony says to Brutus:
In your bad strokes, Brutus, you give good words.
Witness the hole you made in Caesar's heart.
Crying "Long live, hail, Caesar!"
Antony is deriding Brutus for being a friend of Caesar and saying he loved him, even as he killed him. Brutus even talked of his love for Caesar at Caesar's funeral.
Cassius throws the insult back at Antony:
The posture of your blows are yet unknown,
But, for your words, they rob the Hybla bees
And leave them honeyless.
Cassius seems to be referring to the sweet words Antony spoke to the conspirators immediately after they killed Caesar, when he was politically maneuvering to be allowed to speak at Caesar's funeral.
Both insults amount to the men calling each other hypocrites.