In Book 2, Chapter 6 of 1984, Winston walks along a corridor at the ministry, and O'Brien lays a friendly hand on his arm. He'd been looking for a chance to talk. He praises an article, but points out that Winston used two obsolete words. He refers him to the 10th edition of the Newspeak Dictionary. It's unpublished.
And O'Brien says he has one and that Winston may borrow it, which isn't illegal. He gives Winston his address on a slip of paper and suggests that Winston come by. Here, O'Brien makes an apparent reference to Syme, but Syme is an un-person, and Winston believes this is a signal representing a crack in the party's reality. This reinforces Winston's belief that O'Brien, an inner party member, is actually a dissenter. He doesn't consider that O'Brien might be trying to entrap him.
Winston's evolution has become clear, and it is progressing rapidly. He's gone from a vague dissatisfaction to the blatant expression of his thought. thoughts in his diary. Now Winston is on the cusp of action. The end was contained in the beginning.
1984 is a literary tragedy after all, and a tragic flaw is a defect that brings about a character's downfall. Winston is not the typical tragic hero. He knows the actions he's taking are likely going to lead to his demise, but he's going down that path now.