Okay maybe it is just that I hang around a lot of online history communities but I swear these 'misconceptions' are things people always say are misconceptions and yet I never see anyone actually make.
I've seen plenty enough of them in my own time, maybe some of them haven't been used on this forum in a while and others are common misconceptions outside of the crowd that knows history. Stuff like decadent Byzantines doesn't show up as much because pop-history doesn't acknowledge the Byzantine Empire was a thing and this site in particular has had an enduring obsession with them for at least as long as I've been a member. But I have seen stuff like people trying to write "the Mayan Empire" (ignoring, as previously stated, that there was no such thing nor is "Mayan" the correct demonym for the people) into TLs and talking about cities that were long abandoned by the time the TL takes place, even using Spanish names for some cities and modern archaeologists' names for some kings. By which I mean, certain names like "Smoking Frog" or "Curl Snout" weren't the actual names of ancient Maya kings but rather the descriptions of the glyphs representing their names that archaeologists used until they could be properly translated.
Also, another misconception that has been rife for the entire time I've been a member up to the present even is that it's easy to set up a colony in the New World. People continually act as if all it takes for Europeans of any era (or sometimes Chinese, Japanese, Malians, etc) to create a functioning colony is to pack a boat full of people crammed up like sardines and point it west. That's not how it worked at all, surprisingly few people ever even ask why certain people should even be trying to colonize the new world, or if they could even plausibly suspect the new world exists, and don't realize that historically there were numerous failed colonies, including the first several. Vinland was not a success. Columbus's La Navidad was not a success. Roanoke failed, and even Jamestown barely scraped by and most of the colonists died. And these were the attempts by people with good sailing knowledge and actual reasons to go to America.
My impression is that Holy Roman propagandists liked to downplay the Byzantines during the Early Modern era but that that perception is largely gone now.
On the other hand, the importance of the Byzantines (and of the Islamic Mideast as well) during the early Middle Ages is vastly underestimated in Anglo-American historiography in favor of discussing minor “kings” squabbling about in Western Europe...
Eh, if anything the reverse is the problem wherein people discussing the early Middle Ages dismiss Western European kings as mere tribal warlords when they ruled rather sophisticated and important kingdoms that saw trade from far afield. Particularly with England, where seemingly even the English seem like they're in a hurry to write off early Anglo-Saxon monarchs as backwater despots so as to paint themselves as plucky underdogs rather than the people who got lucky and got their hands on one of the most important cities in Christendom very early on.