There are a number of points where Matthew and Luke agree against Mark in their reading, but which cannot be explained by their supposed reliance on the Q source.
For instance, in the narrative of Jesus' trial before the Sanhedrin Mark records:
Then some began to spit at him; they blindfolded him, struck him with their fists, and said, "Prophesy!" And the guards took him and beat him.
While Matthew and Luke add another detail to what was said:
Then they spit in his face and struck him with their fists. Others slapped him and said, "Prophesy to us, Messiah. Who hit you?"
The men who were guarding Jesus began mocking and beating him. They blindfolded him and demanded, "Prophesy! Who hit you?"
In reading a book by Ulrich Luz, he offers two possible solutions: that either there is another lost recension of Mark that was available to both Matthew and Luke or that Luke had access to Matthew but only used it with rarity as a source. He prefers the first option; but frankly, I find neither terribly satisfactory.
Are there other solutions to the so-called "minor agreements" offered by proponents of the two-source theory?