Das Cabinet des Dr. Caligari, the movie made in 1920 by Robert Wiene, featuring, among others, Lil Dagover and Conrad Veidt. One of the most important movies ever. The end is, of course, controversial. However, it is just the ending frame that eliminates all certitudes: are we what we think we are, or is it our world just a gigantic laboratory and we were created just to be used as guinea pigs in horrific experiments? Many books and movies have put this question, I think Caligari was the first.