You are correct Don, that this "touching the mountain" stamp is a printing shift.
The designers did not leave much room for error in the registration of the ink that is used for both the plane and the text labelling the stamp across the bottom. The one on the right has a shift downward from the intended position. If it is down enough that the plane's wheel touches the mountain, it becomes PA68b .
And my example of the regular stamp on the left has a slight upward shift. It doesn't matter so much with with the plane moving up a bit, but the text of "France" is now touching the design. It should be just clear of the design, with white showing between "France" and the design.
The "touching the mountain" is a little difficult to find, but used and MNH examples show up occasionally on Delcampe that will cost several times what a regular MNH runs. That's where I got my example which I found fun enough to work into my AlbumEasy airmail album. There appear to be three up on Delcampe now - one is listed very high at about 17x regular, one about 10x the regular, and there is an inexpensive used one.