Jocasta, Oedipus' Mother/Wife contributes strongly to his downfall in a few scenes and in different ways. One of which can be seen on page 201 in her long speech:

"Listen to me and learn some peace of mid:
no skill in the world,
nothing human can penetrate the future.
Here is proof, quick and to the point."

She continues to ramble about how the prophecy came to her first husband Laius and told of the tragedy of their son Oedipus. She continues to disprove this prophecy by saying Laius was "killed by strangers". She thinks she is doing well by reassuring Oedipus of his safety, however she is only giving him false hopes. The dramatic irony in this moment is overwhelming, since at lines like:

"There, you see?
Apollo brought neither thing to pass. My baby
no more murdered his father than Laius suffered."

She is stating the exact chain of events that occurred, which of course the audience is aware of. By saying this however, Oedipus is enlightened (finally) and recognizes the story told to him in Corinth. He follows her speech, by unveiling his fate in front of everyone.
If Jocasta had never spoken, the true origin of he prophecy would have never been told, and Oedipus would not have made a connection. Because of this Jocasta is responsible for the downfall of Oedipus. - Yuval J.

Jocasta herself should have been able to make some kind of connection ahead of time. Before she even married Oedipus I think she must have remembered what the oracle said long ago. From the start, after hearing that her husband was killed she must have thought of the oracle if even for just a moment. Then soon after her husband is killed she remarries? I think that Jocasta should have been more careful from the start, I think she could have discovered out the truth before marrying Oedipus. In this way her ignorance contributes to Oedipus’s downfall.

I personally fail to see how the fact that Jocasta gives Oedipus false hopes leads to his downfall, so i basically disagree with yuvals statement.
i actually belive that even though she defiles the Prophecies (and gods) as it is clearly shown in lines 1002 and 1003
"He wont admit that the latest prophecies are hollow as the old"

and also in lines 778-784
Jocasta: "a prophet?
well then free yourself of every charge!
listen to me and learn some peace of mind:
no skill in the world,
nothing human can penetrate the future.
here is proof, quick and to the point"
and then tells him about the prophecy which was given to her and her husband years before

that this had little impact on Oedipous as it is clear that he would have found out at some point that Jocasta was his mother and Laius (the man who he killed) was his father. (Niv)

first to Yuval. at that time oedipus did continue on in stateing his prohecy but he at this point in time decided and Jocasta agreed that it was a coincidence and though it was disturbing they didnt really think any more of it until they found the shepard who said that he had given oedipus away instead of killing him like he was supposed to and basically told them botht everything that had happened and the whole truth. next to Beka the reason that she didnt think of the prophecy when her husband died and when she was about to marry Oedipus was that she didnt think it was at all possible that her son could have survived and it never even crost her mind that something like that could possibly have happened. she just thought that it was a sad thing that her husband died but now it was time to celebrate her moving on and getting married to someone new.

She contributes to Oedipus’s downfall by not believing in the Oracles prophecy and killing her child. If she were to have killed her child then there was no way that he could have done what the Oracle prophesized. Also if they would have just raised Oedipus from a baby then they could have told him about the prophecy and he wouldn’t have done it, but instead they make him think that other people are his parents and he doesn’t want to do that to them. – Nate Hamilton