Indeed, Othello described the use of magic in 3.4, where he wanted to scare Desdemona and observe her reaction. But he denied the use of magic twice in this play—first in front of the Council and Brabantio (1.3), second in front of his wife's dead body and all characters in Cyprus (5.2).
And she did gratify his amorous works
With that recognizance and pledge of love
Which I first gave her: I saw it in his hand,
It was a handkerchief, an antique token
My father gave my mother.
You see the difference: in 3.4 the handkerchief was given by his mother, to "subdue my father" (3.4.36); but here, when he was describing the crime of Desdemona, he spoke the opposite—which is the truth?
In my opinion, and according to the Spirit of Tragedy, the description of magic is deceptive. Because in 3.4 Othello was already poisoned by Iago, and he wanted to gain his "satisfaction" (3.3) by scaring Desdemona—and Desdemona, in her fear, did not answer the whole truth.
Most veritable, therefore look to’t well.
Then would to God that I had never seen’t!
Why do you speak so startingly and rash?
Is’t lost? Is’t gone? Speak, is’t out o’th’ way?
Heaven bless us!
It is not lost, but what an if it were?
I say it is not lost.
Fetch’t, let me see’t.
Why, so I can, sir; but I will not now.
This is a trick to put me from my suit.
Pray you, let Cassio be received again.
Mrs. Jameson said "she was betrayed by her fears into a momentary tergiversation.", but Furness suggested that Desdemona did not tell the lie, "she did not believe it to be actually lost, irrecoverably gone; it was merely mislaid ... If she had not been terrified she might have told all this to Othello ... but, as it is, I think in her soul she believed she was telling the truth."
Whatsoever, 'tis fair that Othello's deception did not lead to the truth, and when he finally told the truth, Emilia brought the Justice:
’Twill out, ’twill out! I peace?
No, I will speak as liberal as the north.
Let heaven and men and devils, let them all,
All, all cry shame against me, yet I’ll speak.
She give it Cassio? No, alas, I found it
And I did give’t my husband.
In conclusion, in 1.3 and 5.2, when Othello was in his peak of heroism, he told the truth, which leaded to his fortune and justice; and in 3.4, he was poisoned by Iago, and thus told a lie about the handkerchief, which "over-excited Desdemona" (Cowden-Clarke), and received a non-truth from his wife, leading to "the tragic loading of this bed." (5.2.363). And the differences of these description represents the mastery of Shakespeare's plotting technique.
Source: A New Variorum Edtion of Othello, edited by Horace Howard Furness.