In the world of 1984, history is continually rewritten so that the "facts" of the past confirm the reality of the present moment, whatever that reality might be. For example, if at the present moment, Oceania is at war with Eurasia, it must appear as if it has always been at war with Eurasia: "All history was a palimpsest, scraped clean and reinscribed exactly as often as was necessary," explains the narration. Further, it's not possible to establish that any "falsification" has ever taken place. All the evidence is scrubbed clean or destroyed. Winston is part of this process, as his job is to rewrite newspaper articles that happen to contradict the current Party orthodoxy.
We learn fairly early in the novel, however, from Winston's thoughts, that "Just once in his whole life he [Winston] had held in his hands unmistakable documentary proof of the falsification of an historical fact." It happened, Winston believes, in 1973, eleven years before the action of the story takes place.
Earlier, in the mid-1960s, Jones, Aaronson, and Rutherford, original revolutionary leaders, were arrested and confessed to murder, embezzlement, and treason -- more specifically, to being on enemy soil on a certain day betraying military secrets to the enemy. At work, however, Winston finds a newspaper clipping showing they were actually in New York the day they were said to be in Eurasia committing treason.
Although he destroys this piece of evidence, Winston is excited by his discovery: he believes a revelation of Party lies would blow the Party "to atoms." He is still naive at this point, as indicated by the puzzled questions he writes in his journal of not understanding why the Party does what it does. Later, he will learn from O'Brien that even the old-fashioned "hard" evidence that he so values doesn't matter: the Party has the power to force people to understand evidence any way it wants them to, just as it has the power to force Winston not only to say, but to believe, that two plus two equals five.
Winston's job at the Ministry of Truth is essentially to rewrite history. He combs through old newspapers altering all "false" stories (i.e. stories that are contrary to the Party line). At one point he discovers clear evidence that three men named Jones, Aaronson and Rutherford, had been forced to give false confessions that they had committed treason. They had confessed to being in Eurasia, turning over military secrets to the enemy, when in actuality they were, as a photo in a newspaper reveals, in New York at the time. Even though Winston is constantly manipulating the truth, he is profoundly affected by this discovery:
But this was concrete evidence; it was a fragment of the abolished past, like a fossil bone which turns up in the wrong stratum and destroys a geological theory. It was enough to blow the Party to atoms, if in some way it could have been published to the world and its significans made known.
Winston was terrified, and quickly threw the paper down the memory hole, where it was destroyed. But the memory of the phot stuck with him. It seems to have been the moment when he first began to seriously doubt the Party.