In response to the theory that editorial fatique can be seen in Matthew and Luke as they copied Mark, here is evidence that Mark used Matthew as a main source. Mark sometimes puts a word in an inappropriate place in a sentence. We would expect when someone is rewriting a text that the original text would have the more logical word order. Copying while trying not to copy word for word forces the writer to change the meaning of the original text very slightly.
1. Compare Matthew 26:22 with Mark 14:19, where began fits better where Matthew puts it:
Matthew: And they were very sorrowful and began to say to him one after another, "Is it I, Lord?"
Mark: They began to be sorrowful and to say to him one after another, "Is it I?"
2. The phrases to the paralytic and take up your mat in Mark 2:9 are out of place and appear to have been taken from their original context in Matthew 9:6.
Matthew: For which is easier, to say, ‘Your sins are forgiven,’ or to say, ‘Rise and walk’? 6But that you may know that the Son of Man has authority on earth to forgive sins”—he then said to the paralytic—“Rise, pick up your bed and go home.”
Mark: Which is easier, to say to the paralytic, ‘Your sins are forgiven,’ or to say, ‘Arise, and take up your mat, and walk’?
(ESV)
3. The word immediately in Matthew 4:22 is in a better position than in Mark 1:20: Matthew has "and he called them, and immediately having left the boat and their father....". Mark changes this to "And immediately He called them, and having left their father..."
Compare Matthew 4:20-22
And immediately having left the nets, they [Simon and Andrew] followed Him (Mt 4:20)
and immediately having left the boat and their father they [James and John] followed Him (Mt 4:22)
with Mark 1:18-20:
And immediately, having left the nets, they [Simon and Andrew] followed Him (Mk 1:18).
And immediately He called them, and having left their father Zebedee in the boat with the hired servants, they [James and John] went away after Him. (Mk 1:20)