Saint Mark says that Christ said to Saint Peter at the Mount of Olives,
Verily I say unto thee, That this day, even in this night, before the cock crow twice, thou shalt deny me thrice. Mark 14:30b
However, this goes against what Saint Matthew says that Christ said in the same instance:
Jesus said unto him, Verily I say unto thee, That this night, before the cock crow, thou shalt deny me thrice. Matthew 26:34
It also goes against what Saint Luke and Saint John say that Christ said (although they are recording different instances in which Christ predicted Peter's denials, as their contexts make clear).
And he said, I tell thee, Peter, the cock shall not crow this day, before that thou shalt thrice deny that thou knowest me. Luke 22:34
Jesus answered him, Wilt thou lay down thy life for my sake? Verily, verily, I say unto thee, The cock shall not crow, till thou hast denied me thrice. John 13:38
The most obvious and popular explanation of this variance is that Saint Matthew edited the words of Christ to make them plainer, as the first cock-crowing was likely a minor crowing, and that Saint Mark's record is accurate. However, this explanation goes against the testimony of the other Gospel authors, and what they record about the actualization of Christ's predictions against Peter.
Is it then possible that Saint Mark was, in fact, the one who doctored Christ's words to fit the fact that the cock did crow twice, although the first crowing was likely minor, and that Saint Matthew's account accurately records what Christ said at the Mount of Olives?
Whatever the explanation may be, it is almost necessary that it involves editing of the words of Christ, since the two quotations are identical with the exception of one word, and are records of the same event.