Scribes and scribal traditions

A recent article by Shilpa Sumant (to be published in the proceedings of WSC 14th) drew my attention to the problem of scribal traditions. A single manuscript might be carelessly written or not, but what shall we think when all the manuscripts of a certain text are uniformly full of the kind of various readings one would certainly label as "errors" due to carelessness? I am leaving aside the case of manuscripts copied in order to acquire merit (puṇya). Does this hint at the fact that manuscripts have been copied in a domestic/ritual dimension, that they were thought of as important tools rather than as "treasures" to be preserved? I expect, for instance, a handbook on how to build amulets to be somehow careless written by someone who just wants to understand what he reads and, hence, does not care for the difference between va and ba (which he would anyway pronounce in the same way). On the other hand, I would not expect important mistakes to occur in such practical books (a lacuna would make a medical passage unusable and surely needs to be filled, at least with a marginal note).