I originally tried to submit this last month but I got mod abused so I'm posting it again now with minor revisions.

Today I was skimming though /r/all in search of the platonic ideal of a rage comic when I noticed that /r/AskReddit had decided that it wanted to have a circlejerk about how pre-modern people were stupid:

What is the closest thing to magic/sorcery the world has ever seen?

Joke's on them though because most of the answers were about how being a programmer makes you Gandalf or something. Anyway, I did find this shinning gem of badhistory further down:

Everyone is talking about modern inventions, and they are impressive, but I like to think about how the Hagia Sophia looked to the people of Constantinople. It was built by the Byzantines at the height of their empire and was undoubtably the most impressive structure in the entire world. Then the empire collapsed and the engineering knowledge that built the church was lost. So for hundreds of years, generations of people lived in a city dominated by a building that no one on the planet understood. When we look at "magic" like iPhones and computers, some part of us knows that somewhere in the world is a group on engineers that understands every little bit of it. But not the Hagia Sophia. For hundreds of years these people worshiped in a building so advanced that they didn't even understand how it stood up.

As I understand it, this post was edited to remove mentions of Rome and replace them with Byzantium. This would have been badhistory because the Western Roman Empire fell in 476 and Hagia Sophia wasn't completed until 537, 61 years later. The problem is that the revision makes this badhistory even worse in a number of ways.

First and most obviously is the timeline. The Eastern Roman Empire didn't fall until the Ottoman conquest in 1453 which means that even if we are as charitable as possible with the poster's claim that "For hundreds of years these people worshiped in a building so advanced that they didn't even understand how it stood up" and assume that people somehow lacked this advanced architectural knowledge until 1653 it would still leave us 27 years after the completion of St. Peter's Basilica in Rome in 1626, which for those of you playing at home looks like this.

Second, knowledge of engineering on the scale of Hagia Sophia was not lost to the West at all after the fall of the Eastern Roman Empire. The Cathedral of Saint Mary of the See, a larger cathedral than Hagia Sophia, was completed in Andalusia in 1507, 54 years later. Additionally, many of the Christian mathematicians, engineers, and architects who were responsible for large scale projects in the Eastern Empire fled to Christian kingdoms in the West after the fall of Constantinople.

The third reason this is badhistory is that it assumes that the architectural and engineering processes that made it possible to build Hagia Sophia where for some reason lost or unknown to the Islamic world in general and the Ottoman Turks specifically. In fact, large scale construction of domed temples and palaces was relatively common in Persia, the Levant, Islamic Anatolia, and the rest of what is now the Middle East. This included buildings like Qal'eh Dokhtar (209), the Sassanid Palace at Sarvestan (c. early 5th century), The Dome of the Rock (458), the Al-Aqsa Mosque (705), the Umayyad Mosque in Damascus (715), and the Bursa Grand Mosque (1399) which is located in the city that was the Ottoman capital before the capture of Constantinople. I'm not saying that the OP came to this conclusion because of casual racism but, well, if the shoe fits.

All of this also ignores the fact that Hagia Sophia went through a major structural refit sometime during the reign of Ottoman Sultan Salim II (1566–1574) which was supervised by Mimar Sinan, whose students would go on to design the Taj Mahal which was, ironically, completed in 1653.

Interestingly, the top child comment to this post is also a different but related type of badhistory:

Reading about stuff like this makes The Foundation series so interesting to me

This one requires a little unpacking.

The Foundation Series of novels, written by Isaac Asimov and initially published between 1942 and 1993, are largely premised on a science fiction interpretation of the fall of the Western Roman Empire as told by 18th century historian Edward Gibbon in his series of books The Decline and Fall of the Roman Empire. While Gibbon is rightfully credited with introducing a rigorous methodology to the study of the Roman Empire, he has been subsequently criticized by modern historians like Norwich and Luttwak for minimizing the role of the Eastern Roman Empire and various non-Roman groups by popularizing the so called Dark Ages as the post-Western Empire status quo. This is factually inaccurate because, as we've already covered, the fall of the Empire in the West did not lead to an irrevocable loss of knowledge and expertise in the Eastern Empire or in much of the non-Roman world. Foundation is still a great piece of science fiction, but it's a flawed historical analogy for the Western Roman Empire, not the Eastern Empire.

I want to thank Daeres, shannondoah, cordismelum, and rysnott_ on the IRC for giving me ideas for this post.