The second most annoying thing in researching medieval cooking - the first of course being the myth that medieval cooks used spice to cover the taste of spoiled meat - is what I usually refer to as a cheeseburger argument. This is usually an attempt to justify a modern dish as being "possibly" medieval, and generally takes the following form:
"They had beef in the middle ages, right? And they sometimes ground meat, right? And they sometimes cooked meat on a grill and they had cheese and bread, right? So they had all the things they needed to make a cheeseburger. Therefore the odds are that someone somewhere made one in the 500 years between 1000 and 1500."
Um ... no.
Sure, it sounds reasonable, but aside from being research in the wrong direction (choosing something modern and looking for it in the past, instead of looking at what there was and trying to make sense of it) the argument contains a number of logical flaws.
One of the flaws is called "Post hoc ergo propter hoc" (literally "After this, therefore because of this"). To make a cheeseburger you need to have all the ingredients (true), so having the ingredients will inevitably lead to making a cheeseburger (false). Thousands of people in the US have raw fish and lye in their houses, and the vast majority of them will never make lutefisk.
This could also be a form of an "appeal to probability" (I still can't find the fancy Latin term for this), where it is assumed that because something can happen, it eventually will happen.
Another flaw present is a "fallacy of composition" (again, no cool Latin ... I'll keep looking), which assumes that if something is true for the parts then it's true for the whole.
So, while medieval cooks had all the stuff they needed to make a cheeseburger, and while it was possible that someone would make one, the concept of a cheeseburger (or any kind of sandwich for that matter) simply isn't one a medieval cook had. Eventually someone did come up with the idea of a sandwich (most historians think it was sometime in the 18th century) and I'm sure a cheeseburger appeared shortly thereafter, so I suppose that given all of human history it was inevitable, but it wasn't in the middle ages.
Sometimes those making a cheeseburger argument tack on a short addendum when challenged, something along the lines of "You don't think they made cheeseburgers? Prove they didn't." In terms of classical logic this is called "Argumentam ad ignorantiam" (argument from ignorance), where the only "proof" that a premise is true is that it hasn't been proven false. If this is allowed than almost anything can be "proven" - Henry the Fifth was actually a female orangutan! Don't think so? Prove he wasn't. See?
Still it's soooo tempting. There's a particularly appealing modern dish, and a recipe or two from the 14th century that has a couple ingredients in common, and maybe even a hint at a method that kind of sort of matches, or maybe just a name that's spelled in a similar way. That's why we need to practice CONSTANT VIGILANCE!
I'm going to go have a cheeseburger now.