When Rosemary has decided to sneak over to the Castevet's, she removes the towels from the closet shelves. She then tries to shut the door by pushing the towels (on the floor) into the closet. We see that the closet door does NOT shut all the way. Rosemary goes to get a knife, when she gets back to the closet, the door is completely shut, and she must now turn the knob to open it.
This is the very spot at the Dakota building on 72nd Street in New York City where John Lennon was murdered.