Focus advice - 50mm 1.8
From the framing (and focus point on the near shoulder in the second image) it would appear the focus didn't change (but the distance increased about 4 inches). As to why it happened, that's a harder question to answer. Was the camera in one shot or AI servo? Was the photo part of a burst of several images? How was the focus point selected? It's possible to move that much after focus is locked (especially when taking photos of a person who also moves slightly).
Stopping down a little will give you more room to get focus right. (and a blue sky is far enough away and low enough in details that how it looks isn't going to change much at any aperture).
When shooting really low depth of field, I prefer to use a single single focus point (ideally one of the cross type points) and place it where you want. AI servo mode will typically help with the minor movements that can occur between focus lock and shutter release as well.
I use back button focus, one shot. This image was off centre, so I used an outer cross type focus point on the 60D.
Indeed stopping down would be fine with this particular background, however this is indicative of the problem, regardless of background.
I'm not sure of your meaning "AI servo mode will typically help with the minor movements that can occur between focus lock and shutter release as well" - I guess you mean when using the shutter button to focus? (which I don't).
I don't think its the glasses (this is where I placed the focus). It happens against a bare eye, too. I'm wondering if the cheap focus mechanism in the lens just can't be accurately relied on at thin dof.
Cheers for your time.