When I'try to mirror Piero's left side eyelids shapekey, I get a distorted mesh on both sides:
This is the left side's shapekey working fine before the mirroring:
The piercing through of the eyeball during the opening will be corrected in the "Rigging" chapter of this course according to Kent.)
My Piero file is here.
This "Shapekeys" lesson with its difficulties in the eyelids opening turns more and more out to be a bottleneck 😉, and I can barely wait to continue with the texturing and the adding of the hair system feathers in order to complete the feathering 😀!
The problem is that your mesh is not symmetrical anymore. Something went wrong between you applying the mirror modifier and making your shapekeys. I know this because when I tried mirroring your shape key it gave me the error that Blender couldn't find perfect symmetry which is why some verts move on both sides.
This can occur if you accidentally move some verts on one side or the other, or if you move the origin of your object (on purpose or accidentally). It's happened to me before and I can't even remember changing anything like that. But that's definitely the problem. Here's what you need to do to fix it: Restore *Perfect* Symmetry
Thank you, @theluthier, for the instructions on how to solve this problem. I was already anticipating that this could be necessary since using "Symmetrize +X to -X" ended in this:
I promise that I will never do this again with poor Piero ✋! And in the meantime it's already good practice to create shapekeys 😉. My goal is to make Piero smile 😊. After this it's time to tailor him a beautifull feathering 😀.
The eyelids opened at the maximum:
I've done the fineadtjustment in "Edit Mode" with "Edge Slides" and the "Fatten" function in order to have maximum control.
With some further adjustments at the eyelids and the smile shapekeys added, the final result can be seen here in the "Duerer's Progress" thread for this lesson.