This Bible contradiction is from the Skeptic's Annotated Bible.

Verses

Peter denied Jesus three times before the cock crowed.

Matthew 26:70 View context

But he denied before [them] all, saying, I know not what thou sayest.

Luke 22:57-60 View context

And he denied him, saying, Woman, I know him not.

And after a little while another saw him, and said, Thou art also of them. And Peter said, Man, I am not.

And about the space of one hour after another confidently affirmed, saying, Of a truth this [fellow] also was with him: for he is a Galilaean.

And Peter said, Man, I know not what thou sayest. And immediately, while he yet spake, the cock crew.

John 18:17 View context

Then saith the damsel that kept the door unto Peter, Art not thou also [one] of this man's disciples? He saith, I am not.

The cock crowed after Peter's first denial.

Mark 14:67-72 View context

And when she saw Peter warming himself, she looked upon him, and said, And thou also wast with Jesus of Nazareth.

But he denied, saying, I know not, neither understand I what thou sayest. And he went out into the porch; and the cock crew.

And a maid saw him again, and began to say to them that stood by, This is [one] of them.

And he denied it again. And a little after, they that stood by said again to Peter, Surely thou art [one] of them: for thou art a Galilaean, and thy speech agreeth [thereto].

But he began to curse and to swear, [saying], I know not this man of whom ye speak.

And the second time the cock crew. And Peter called to mind the word that Jesus said unto him, Before the cock crow twice, thou shalt deny me thrice. And when he thought thereon, he wept.