Just finished reading Dan Brown's Inferno while in Italy. I know I should not bother correcting stuff, but for this book he declares at the beginning, "All artwork, literature, science, and historical references in this novel are real." So, he is inviting closer scrutiny than in earlier books, where he definitely stretched and invented and distorted lots of books, but avoided such claims. Here are some errors of fact in this book (which is a fun read anyway, although formulaic).
p. 95: "To this day, modern banks use the accounting method invented by the Medici - the dual-entry system of credits and debits." Wrong. It was first used in Pisa where I sit at this moment a good two centuries prior to the Medicis getting into banking. BTW, the oldest continuously existing bank in the world is also here in Tuscany, the now-troubled Monte dei Peschi di Siena, where its HQ still is since it was founded in 1472, which is the period of the Medicis. [Correction: Double entry accounting was not invented in Pisa, but in Florence. However, it was initially done by merchants in the late 1200s prior to the Medici, although they may have been the first to use it in banking. Brown is not as far off on this matter as I initially suggested.]
p. 96: "You can't get into the [Boboli] gardens from here. The entrance is way over by the Pitti Palace. You'd have to drive through Porta Romana and go around." False. From where they were standing just outside of the Porta Romana, southern entrance to Old Florence, there is an entry to the gardens just to their right, a few meters away, not requiring going through the Porta Romana.
p. 103 (and related pages): A graph of world population is shown starting 2 million years ago, which, with this scale highly magnifies recent growth while not really showing that the growth rate of world population has been decelerating since the mid-1970s. It is repeatedly declared, including by a character with an IQ of 208, and never questioned, that it is a "mathematical certainty" that exponential population growth is simply continuing at a constant rate with no mention of the slowdown. It is mentioned that world population might reach 9 billion by 2050. The latest estimates are that this level might be reached later and might be the maximum world population will ever reach. Population growth is almost certainly going to go negative before the century is over due to the spread of falling birth rates across more and more countries, which is happening.
p. 106: "If we head northeast, we'll reach the palace." From where the characters were at that point (having entered Boboli Gardens by jumping over a wall near the southern entrance), the palace is northwest, not northeast. The error of the location of the palace is repeated throughout the coming pages. It is on the northwest end of the gardens, not the northeast.
p. 136: an even more ridiculous figure with rates of growth of various variables stuck on top of each other so that they all appear to converge at a point at the right end of the figure. Some of these are extremely misleading. Thus, one is the rate of growth of the demand for fresh water. Yes, that is growing, but the rate of growth of the supply of clean fresh water has been growing more rapidly as a rising percentage world population obtains clean water. I will note that some of the variables do reflect real problems, such as species extinction and overexploited fisheries.
pp. 141-142: He has the Vasari corridor starting at the northeast end of the Boboli Gardens and going east across the Arno River. It is at the northwest corner (near the palace) and goes basically north and a bit west across the Arno.
pp. 144-145: "A quick Internet search led him to information about a prominent nineteenth-century mathematician and demographer named Thomas Robert Malthus, who predicted a global collapse dut to overpopulation." I do not think anybody has ever described Malthus as a mathematician, although arguably he was a demographer. In any case, besides being a minister, he was the first Professor of Political Economy in Britain, and he did not predict "global collapse." He did predict ongoing poverty and stagnation due to overpopulation, but not this more dramatic outcome. That would come with some of his followers, particularly in the 1970s, when this book would have been more timely.
There are numerous repetitions later of the inevitably of global collapse due to overpopulation, accompanied by such statements as "The mathematics is indisuputable" (p. 214). Nowhere does Brown ever let any character remotely bring up the numerous well known issues questioning this that are mentioned above.
p. 294: "In the 1940s, Nazi scientists had dabbled in a technology they'd dubbed eugenics..." The term was invented decades before the Nazis and was highly respected and supported by many progressives in the US and elsewhere. It was the Nazi embrace of this already dubbed and established eugenics that made it become a disrespected movement.
p. 324: "Architectrually speaking, the word basilica defined any eastern, Byzantine-style church erected in Europe or the West." The first buildings to which the term was applied were built in Rome during the Roman Empire period, prior to the Byzantine period. The term moved east initially, and more generally is a church term tied to having a saint buried or strongly associated with the building
[Addition: The first basilica was built in 184 BCE in Rome by Cato the Elder. They were originally public building located in the forum of a major Roman city. Their crucial architectural detail was the presence of colonnades in the interior to create separate spaces, not a dome, although some do have domes.]