I am quite unlikely to ever read this, but I would love to hear of some medieval tropes which are hilariously incorrect.
I especially like this video: https://youtu.be/lZfpQzV2GnM ("Ropes and Hollywood", a rant about the value of rope and the silliness of cutting one 1-4 times every 90 minutes)
[1]: https://www.youtube.com/watch?v=ViV6VmI9g0U