Atlas was kind enough to note the exist of Ars Magica Wikia a year a so ago. Please feel free to come and take a look and add any contributions.
But I'm posting for contribution to a specific page- List of Anachronisms. It's meant to list the kinds of things that are, may be, or you might think are, but really aren't, anachronisms for Mythic Europe (around 1220 AD).
I've put in a few terms and one holiday practice (Christmas trees, not really in use in the 13th century). Most info, but not all is from Wikipedia or general info. In the long term, I'd like to even see distinctions for regions- such and such is practiced, but only in this isolated location.
This is not Wikipedia, so it's not as anal about citing, but try to find some sourcing or basis for what you put in, when possible.
The specific page is http://arsmagica.wikia.com/wiki/List_of_Anachronisms. If you just want to take a look around, go there and then navigate to where you're interested.