"Companion to the Roman Army"

[D B Campbell]

... freelance copy-editors ... are not necessarily even interested, let alone specialists in, the subject and could never pick up an error of fact like this

... which is why publishers like Blackwell commission "expert" editors, who are supposed to invite trustworthy contributors on board.

Well, mistakes happen, it's almost impossible to avoid them.

Indeed. This is why most conscientious authors attempt to have a colleague read over their work prior to publication! (Not always possible, I know ...)