Naturally different scribes had different hands; that’s of course one of the ways we can tell how many scribes worked on a manuscript. And hands differed across Europe. Naturally. And sometimes scribes had to copy passages in languages they did not know, though scribes were trained, and never unlettered. And MOST certainly all sorts of mistakes got made in copying. Chaucer wrote a poem about how annoying that is.
But as I said up at the top of this thread, mistaking an s for an n will not have been the problem.
An example of a MS in Norman French, from the 2nd half of the 12th century:
https://www.bl.uk/collection-items/philippe-de-thaon-comput
On the second line down, the 4th word is “desquassee.” You can see the long “s” form, typically used in the middle of words. At the end of that line is the word “enfundree.” As you can see, the u and the n might easily, in a sloppier hand than this, be confused. But the n and the s are totally different.
An example of Anglo-Saxon, from the late 11th century:
http://www.bl.uk/manuscripts/Viewer.aspx?ref=add_ms_34652_f002r
Nine lines down, the fourth word is “cyning” so there is an n. The next word is “as” so there is an s. Not confusing.
A snippet from the Domesday Book, so, a Norman scribe writing in Latin:
http://www.nationalarchives.gov.uk/education/focuson/domesday/take-...
This is highly abbreviated, as is the norm, but if we’re just looking for letter forms, 8 lines down, the 4th word in is spelled (not counting the abbreviation) (this makes me nuts, but I don’t want to complicate things) “Pasta” so there is the s — and then two words later you can see “pecun” and is there is the n.
Anyway, I haven’t heard of “s”es and “n”s being confused. Lots of other stuff, but not that.