I was following the Projection video tutorial where you take the 2D image of a green field with a treeline in the background and create a 3D camera move.
When I duplicated the "Source.png" layer, and renamed it to "Sky", it automatically changed the source in the Ground projection source to the "Sky" image. I didn't notice this until I started to move the "Sky" layer back to place it behind the trees. As I did this, the ground plane started changing. This did not happen in the video.
So, after duplicating and renaming the Source.png to "Sky", I had to go back into the Projector properties of the Field layer and change it 'back' to "Source.png". I did not remove the Source.png layer from the composite. I don't know if this qualifies as a bug.