When does criticism of a writer's work cease to be reasonable and become nit-picking?
In a novel set in 1797, two army officers are returning, on horseback, to Woolwich, from a country house in Kent, described as being forty miles from London, south of the Maidstone road. They are riding up Shooter's Hill.
They halted at the top of the hill beside a curious, triangular tower....
'What is this thing?' asked Whittington, looking up at the tower....
'It's a memorial to a man called Sir William James..'
Except that Sir William's memorial, Severndroog Castle, isn't at the side of the road as one goes over Shooter's Hill, or even, nowadays at least, visible from the road. I can't help feeling the author has confused it with another tower, an early twentieth century water tower, which actually is at the side of the road at the summit of Shooter's Hill. (Google brings up yet a third tower, the water tower of the former Brook Hospital just beyond Shooter's Hill to the west.)
Aside from this confusion of towers, if these two gentlemen were returning to Woolwich via the Maidstone road, they should not have been on Shooter's Hill at all.
Does any of this matter? Only a few readers would pick up on these points. None of it has any impact on the plot. The point of the journey was to provide an opportunity for the two characters to have a conversation, which they could as easily have had at an inn while eating and resting their horses. The mention of Severndroog doesn't seem to serve any plot or character related purpose at all. While I was briefly thrown out of the story, my overall enjoyment of the novel wasn't spoiled.
On the other hand, while these might be minor nitpicks, they are errors that need not have been made. Maps, both current and historical, are easily available, in print and online, for working out journeys. Sometimes travellers' descriptions of particular roads exist. If this author had wanted a near-contemporary account of the Maidstone road, she could have referred to the Torrington Diaries of 1781-94.
But how far is it necessary for an author to go in researching these minor points? Do small mistakes cause readers to lose confidence in an author's research into major plot elements even if, as in the case of this novel, it appears to be thorough and detailed? Of course novelists should aim for accuracy in what they write, and a failure in research which rendered a plot implausible or impossible would matter. But readers should probably be forgiving of minor errors, as long as a book does not contain too many of them.