This is a great topic. I will start with a well-documented error in Julia Lovell's book
, from the first photo section:
"Lieutenant William Parish busied himself producing equally fanciful and romantic paintings of the wall festooning bosomy hills as far as the horizon stretched, interrupted by artistically ruined towers, their square stone edges becoming frayed."
One only has to go to Gubeikou and look from the Panlongshan Wall back across the river to Wohushan to see this painting is not fanciful at all, but actually quite accurate. And, William Parish was a Captain, not a Lieutenant.