Bill Heroman has recently written a post that draws attention to the heuristic distinction between contrast and contradiction (my language, not his. I'm incidentally following Robert Doran in this verbiage). A contrast is an instance in which A and B are irreducibly different and can both be true, whereas a contradiction is an instance in which A and B are irreducibly different and cannot both be true. Bill's discussion focuses upon the differences between the Matthean and Lukan birth narratives, and (to use the language employed here) he notes that while they contrast significantly the only contradiction is where the holy family travels after Jesus is born. I think that he is probably correct on this.
Let's approach this a bit more schematically. Take the example of who visits the family after Jesus is born. Matthew reports that magi come from the east, following a star in the sky. Luke reports that shepherds come, following the instructions of angels. It is an irreducible difference: magi cannot be made into shepherds or vice versa, nor angels into a star. But it is a difference that is not mutually exclusive. Both could be true. Magi could have visited, and also shepherds. Of course, one or both could be false, but that's not the point. The point is that it is a logical possibility that both are true. As such, this is a contrast.
But with the matter of the holy family's travel itinerary, Matthew has them leaving Bethlehem in the middle of the night to flee to Egypt, only to return to Nazareth years later. Luke has them travel from Bethlehem to Jerusalem at a leisurely pace, then settle back in Nazareth. These are not only irreducibly different but also cannot both be true. Affirmation of one excludes affirmation of the other. Again, it could be the case that both are false, but the possibility of both being true is foreclosed. As such, this is a contradiction.
This might seem like a trivial point, but it is something that tends to bedevil biblical scholars. Precisely because contradictions close off the possibility that one can affirm both at the same time, biblical scholars who misidentify contrasts as contradictions have excluded a logically possible hypothesis from the off. In some cases, this will make no difference whatsoever, not least of all because ultimately historians are not in the business of affirming statements ready-made in our primary sources, but in some cases it will. Those will be the cases in which both texts provide generally accurate accounts of what transpired. We cannot know before getting into the weeds whether these cases are the exception or the rule. Programmatic decisions to treat all contrasts as contradictions (the skeptic's erroneous tendency) or all contradictions as contrasts (the credulist's equally erroneous tendency) will tend to obviate genuine historical knowing.