I know that many early Christian scribes were willing to make changes, mostly small, to reconcile differences between different biblical accounts and to resolve theological points. I also know that some early Christian leaders, such as Marcion, were willing to make larger, more significant alterations.

Reading, for example, the birth narratives in Matthew and Luke it is clear that there are many differences. Of course some people argue that they are not irreconcilable, but surely this must have been apparent even to early (100-400 CE) Christians. So, why were small cases of 'harmonizing' or altering texts common, but not large cases? Were early Christian scribes not seeing the forest for the trees, or did these contradictions not bother them? Or were there 'large cases' of harmonizing that I am not aware of?