Anything about science and education that begins "During the Dark Ages..."

"People died so young in the Middle Ages that somebody who lived to be 30 would be considered to have died an old man."

This one is more often than not based on one person misunderstanding how to determine life expectancy and a bunch of other people repeating it mindlessly because it sounds funny. This one isn't exclusive to study of the Middle Ages and I'm sure it's gotten asked about on this subreddit a hundred thousand times.

"People in the Middle Ages never, ever, ever bathed."

This one can be chalked up to people viewing medieval society as a total monolith across regions/occupations/class divisions and indeed across centuries. "Medieval people never bathed" would come as news to any medieval Jewish or Muslim person, or to a 13th century Bohemian with his hair up in a towel, or to a 14th century Flemish bathhouse operator and her staff. Did people smell bad? They sure did, especially to people from cultures or classes that bathed more frequently. Did everybody just walk around caked in pigshit constantly without a care in the world? No.

"Infant mortality was so common that people in the Middle Ages didn't care about or feel emotional attachment to their offspring.

This one makes me want to scream. At least unlike the above misconceptions, it's a product of actual scholarship, even if that scholarship is dead wrong.

"During the Middle Ages (that's right, the entire Middle Ages, from the 5th century to the 1550s) everyone got married really young, probably because everybody died at 25." Or its evil twin, "During the Middle Ages, marriage was an institution exclusively binding together prepubescent girls and horny old men in legally sanctioned child sexual abuse".

No, wait, this one makes make me scream.

Really, any statement you make about what everybody did "in the Middle Ages" or in medieval Europe risks being an overgeneralization -- location, class, religion, era, occupation, gender, and so on really do make a massive difference, and part of the study of history is just drilling down to a more fine-grained understanding. This doesn't mean that high-level assertions or questions are necessarily bad -- it'd be hard to go through study of the Middle Ages if you only talked hyper-specifics and never what was likely or common. But when it comes to broad assertions that people just take for granted as facts of generic medieval life, always and everywhere, there are some frustrating re-occurrences.

People often think of medieval Europe as a poor, cut off backwater. This isn't true at all, especially when you consider the accomplishments of different "countries" through the time period. I will largely be focusing on military and strictly political history here since it's what I'm most familiar with.

One misconception I hear commonly is that medieval kingdoms where incapable of maintaining militaries and this made them "easy pickings" for the Vikings. While this can be considered partially true for a while, during the reign of Albert the Great of Wessex in the 800's CE the Kingdom of Wessex maintained a permanent army of over 27,000 men stationed in fortified burrows throughout southern and central England to defend against raids and the rather confusing Danelaw to the north. This all required an incredibly efficient top down bureaucracy to assess land value and population, something that people don't think of when they imagine medieval Europe.

Something else that really irks me is that people tend to think of medieval Europe as one long time period with little change and overall stagnation. This is entirely false. For example, most people I've talked to think that medieval Europe started in the year 476 with the "fall" of the western Roman Empire to the Goth Odoacer and ends sometime in the 16th century. In the late fifth and early sixth centuries there were roughly six major groups in mainland Europe and the Mediterranean. These were the kingdoms of the Vandals (North Africa), Ostrogoths (Italy), Visigoths (Iberia/Southern France), Franks (Northern France), Eastern Roman Empire (Balkans/Middle East), and the Alemanni Confederation (Rhineland). In the year 1400 the Eastern Roman Empire only controlled Constantinople and some scattered lands around Greece, all of the other kingdoms had either completely disappeared or formed different political entities. Instead of the dozen or so countries on mainland Europe there were hundreds of political units, many of them overlapped each other. People don't realize that human society has never stopped changing even though the middle ages and there has never been a time in human history where things have simply stopped.