RICHARD CHAMBERS said
When I read about the theory of the Q documents, a proposed source for the gospels of Matthew and Luke besides Mark, I always wonder about how it explains the story of Peter denying Jesus three times before the rooster crows twice. The Q theory would suggest that the Q material was only about sayings of Jesus, not stories.

Both Matthew and Luke say he denied Jesus three times before the rooster crows once. If both gospels used Mark as the main source for the passion narrative, one would expect them to say Peter denied Jesus three times before the rooster crows twice. Was there another passion narrative source? Or the story in Mark changed later, after the other gospels used it? Or did Matthew and Luke coincidentally both change the source’s story in the same way?

Was the Q document about more than sayings? Is Q theory even the best explanation of source material patterns?

  

Excellent question.

There’s a simple solution here: Q doesn’t exist as an independent, stand-alone document; and Luke most likely changed the rooster crowing based on Matthew (which Luke had access to), but also possibly in concert with another now lost source or sources, which likewise said once.