Here are some phrases and words that I tend to trip over:
"Scanning" a room
My brain was "fried"
"train" of thought
"Pause" and "Rewind"
It makes a lot of sense in retrospect that medieval characters wouldn't know about these things, but they are engrained in my mind, seeing that I live in the twenty-first century. The two main subjects that writers should look out for are references to trains and computers. It is surprising how often those two pop up. The list is much much longer than my sample, but it is good to keep this concept in mind for the future–er–I mean, for the past.