By the bye, this picture was taken with a 50mm lens, a Leica Dual-Range Summicron on an M4. I was originally planning on asking whether it was taken with a 40mm or 50mm, but I thought that the answers might divide 50-50 just based on random chance. Despite this, if you look at the survey numbers as of tonight, you'll see that although the consensus wisdom was correct overall, "50mm" and "not 50mm" did indeed divide up almost exactly 50-50—49.5% voted for something other than a 50mm and 50.5% thought it was a 50mm. And this is despite the fact that all the choices were more than a 15% divergence from the actual focal length.
This is just one example, of course, but in my experience it's much more difficult for people to make many kinds of distinctions when they only evidence they have to go on is the visual evidence in pictures.