At one point Christina removes her dress then jumps from the dock into the water. Several wide shots show the entire dock and several feet of ground leading to the dock. Unless she tossed the dress into the water it should be seen in the dock, but it isn't.
When David sees the apparition in the gazebo, the state of day/night changes repeatedly in the ensuing shots (before he falls into the lake).
There are obvious body doubles for both Christina's and David's nude scenes. You can see their faces clearly - most notably, Christina's double's face can be seen through the tree's leaves when she emerges from the lake. David's double's face is clearly seen during the sex scene with Christina.
When Christina is playing the piano, none of the hammers move.
When the gas lamp explodes upstairs and fills the hall with fire and smoke, part of the ceiling flips up from the explosion and slips back into place, revealing the location is on a set.