Liesl creeps through the window and explains herself to Maria, standing by the middle of the bed. When the angle changes she is by the side of the bed. Then the angle changes and she is back by the middle side.
The front of the von Trapp house and the back of the house are actually two different houses. Also, the gazebo (which belongs to the house used for the back shots) is actually across a river from the house. (You can take the Sound of Music tour in Salzburg.)