Not the first horror film shot in a single take
The horror film INVASION aka INFECTION that I wrote and Albert Pyun directed, was filmed in one single unedited shot in 2005. Lionsgate released the film in 2007
http://www.imdb.com/title/tt0472465/
"Cut" is a bigger film than ours and I'm looking forward to seeing it, but it's not the first single shot horror film.
Cynthia Curnan