One of the rules of thumb of modern textual criticism, is that a scribe is more likely to insert than omit. This rule of thumb needs to be reexamined.
1. Methodological Considerations
The rule of thumb does not appear to have been derived empirically. Instead, the rule of thumb seems to have a popular solution to the dilemma that often faces textual critics: did document 1 add or did document 2 omit?
The supportive reasoning is usually that scribes would have been timid to leave anything (even marginal notes) out, because they would not want to subtract from the Holy Writ. Furthermore, there are occasional examples that can be identified in which a scribe apparently inadvertently copied a marginal note into the text.
Another supportive reasoning is that scribes are more likely to explain something hard than to omit the explanation of something simple. Thus, this reasoning argues that scribes are more likely to provide an explanation than to omit it.
Neither of these tenets, however, appears to be readily testable. The warning against subtracting from Scripture is paired with a warning about adding to Scripture. Furthermore, a hurried scribe might omit an explanation that seemed obvious to the scribe. Thus, neither supporting reasoning is motivationally compelling. Neither supporting reasoning is the result of a method of analysis designed to figure out what errors actually do occur, in order to decide what default rule to use in questionable cases.
2. Empirical Considerations
The known subtractions/omissions heavily favor subtractions as the default error. There are few cases that we can identify where a "sleepy scribe" (as the 19th century collators called them) had inadvertently copied in a marginal note. On the other hand, homoioteleuton is a frequent scribal error.
On top of that, it is clear that transcribing texts by hand was tedious. Taking shortcuts rather than long cuts is a well documented observation of human nature.
Finally, especially early on, marginal material was clearly distinguishable, falling outside the relatively neat columns of letters, and varying in size and hand from the original. In some cases there may have been troubles to distinguish between corrections and commentary, but the unusual situation is more likely to have made the scribe pay attention than doze off.
In view of those two considerations, the present author takes a different default position, namely that all things being equal, it is more likely a careless scribe omitted than that a careless scribe inserted material.
This different default position happens to change the preferred reading of many passages from the modern critical text back to the Textus Receptus, restoring (in this author's opinion) the original fullness of the text.
May God Greatly be Praised,