|How many times did the cock crow?|
Three gospels agree; one does not, supposedly. Only Mark says that the rooster would and did crow twice. In order to examine this claim, though, we need to look at the verses of concern and offer some textual-critical data -- because there are actually three we need to talk about.
Mark 14:30 And Jesus saith unto him, Verily I say unto thee, That this day, even in this night, before the cock crow twice, thou shalt deny me thrice.
Mark 14:68 But he denied, saying, I know not, neither understand I what thou sayest. And he went out into the porch; and the cock crew.
Mark 14:72 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.
The textual tradition is not unanimous for any of these verses. In all cases, some say twice; others lack it, and there is a mix of combinations.
But what it runs down to, in terms of weight of evidence, is that 14:30 and 14:72 are likely to have been part of Mark originally, whereas the key verse in 14:68 ("and the cock crew") is not, and was likely added to make the fulfillment of Jesus' prediction more exact.
That said, what of the fact that the other gospels do not say "twice"?
Strictly speaking, there is no contradiction in action, since of course if Peter denied before the cock crowed once, he also did it before the cock crowed twice. In that light, I would suggest that Mark offers the original verbiage of the prediction (as might be expected, if Mark is recording from Peter), while the other gospels contain a modified and simplified oral tradition that follows the usual oral-tradition pattern.
Bear in mind that within this context, this is not considered "contradiction" or "error" -- no ancient reader would have thought this. Compromises in narrative presentation were often necessary to make a text more memorable for a population that was 90% illiterate. Intentional, structured changes for a purpose are not, under such a semantic contract, an error.
Keener's Matthew commentary  adds a salient point: A cock's crowing lasted as long as five minutes and occurred at all hours; as Cicero wrote: "Is there any time, night or day, that cocks do not crow?" The "second" cockcrowing was usually associated with the dawn.