If you've ever been to Cairo, you'll notice there's something not quite right about Poynter's Egyptian landscape (pictured below) - he's managed to pack in too many great buildings that are in truth spread very far apart.
And it's argued Poynter vastly underestimated the number of slaves you would need to haul around a big stone sphinx (accurately, the Sphinx in this painting still has its nose - a fanatic sufi in the 14th century is blamed for at least partially de-facing it, and there is also a good hypothesis about vandalising Mamluks, but I prefer the theory that Napoleon's troops shot cannonballs at it - it has that sort of "I got hit by cannonballs look to it. I suspect the truth is all the stories are true, but then I believe in the Broken Window Theory").
Andrea Fernandes has more stories about this painting on the Mental Floss site here.
Personally, I found wandering around the pyramids a bit eerie: here was an empire so powerful that it managed such a feat with the tools of the day, and it makes me wonder whether in 3000 years a new empire will have even better tools, and be putting up ugly pre-fab houses and look at our then-ancient NYC skyscraper ruins and say "Can you believe those Americans did that with those puny cranes and 20th century steel?"