yalla

1.The gospel according to "Mark", 14.72 [RSV]

"And immediately the cock crowed a second time. And Peter remembered how Jesus had said to him, "Before the cock crows twice, you will deny me three times." And he broke down and wept."

2.The gospel according to "Matthew" 26.75

"And Peter remembered the saying of Jesus, "Before the cock crows, you will deny me three times." And he went out and wept bitterly."

3.The gospel according to "Luke" 22.62

"And Peter went out, and wept bitterly."

I am offering this as evidence as one more of many instances where, presuming "Markan" priority, either "Matthew" or "Luke" has copied the other and that the hypothesis of Q is unnecessary.
The "Peter wept" scenario is part of "Mark" and, IIUC, not considered to be part of Q. So the similarity of the bolded bits is, IMO, due to one of "Matthew" or "Luke" copying the other and not both relying on an unknown hypothetical document Q.
Both use "went out" and "bitterly".
cheers
yalla

Toto

Chili-Gibson digression split off here

andrewcriddle

Luke 22:62 is missing in a few ancient manuscripts mostly Latin.

Possibly it was added to original Luke based on the text in Matthew.

Andrew Criddle

Solitary Man

April DeConick recently spoke on this topic. Essentially, when working with Matthew/Luke, the possibility of influence from copyists (proven elsewhere) has to always be considered. Other considerations include Deutero-Mark/Ur-Mark nor can we exclude Mark/Q overlaps (and the probability that our knowledge of Q's content is deficient).

As DeConick said, essentially we have a 3rd century text with some portions going back to the 2nd century. This is why textual criticism is an on-going investigation, and why Q hypothesis still exists - in my opinion, overall Q works better as a theory, even if it doesn't for every example, but neither does Matthew using Luke, or Luke using Matthew.