https://en.wikipedia.org/wiki/The_Man_Who_Came_Early
Another classic one is "Lest Darkness Fall" by L. Sprague de Camp in which an archaeologist is transported to Rome in the year 535 AD.
On that note, Wikipedia took me to "The Man Who Came Early" by Poul Anderson, which fits what I'm saying exactly: a modern day man is taken back to Viking-age Iceland, where his knowledge fails him catastrophically as he is unable to explain it or put it into practice, to disastrous results.
