I can understand the confusion some people have between Mary Tudor and Mary Stuart. First, they usually get called by nicknames, and second, their lives overlapped a fair bit.
I can also understand confusions people have between the Hundred Years War and the Thirty Years War. Both lasted a long time, and they're both a long time ago.
I can't get my head around my student's confusing the two World Wars of the past century. Now, I could probably understand if this were a sophisticated argument about how the wars and such of the 19th century provided impetus for the Great War, and that WWII was an inevitable result of the resolution of the Great War and so on. But no, this student just conflates them and puts them both in 1918 with Hitler at the head of SS troops and such.
The same student confuses Modernism (as an intellectual and social movement/practice) and modern (as in what's happening now), which I totally understand, but since I've tried to explain that Modernism is a pretty specific movement, and it just hasn't gotten through, I'm finding frustrating. No doubt my own deep sense of the difficulty of really understanding Modernism has gotten in the way.
What are your students' most confusing confusions?