I've been thinking on and off all day about my worst scene. Horror doesn't scare me, some scenes (like the needle one) make me squeamish but don't actually worry me.
But I've remembered the scene that had me thinking about it for days in disbelief. In the film 'O Lucky Man!' with the magnificent Malcolm McDowell, there is a scene in a clinic where he comes across a man/pig hybrid. I remember it as being a man's head on a fat wobbly pig's body, he spoke to Malcolm and I was just so shocked at the whole scene.
It was made in 1973 and such things were the stuff of science fiction then, nowadays with cloning, pigs hearts being used in humans, growing ears on mice and face transplants, it's becoming science fact.