X-Pro2: Strange effect in a heavily back-lit shot

Yesterday, I took the following shot under heavy back-lighting (I know, this isn't a work of art, so I'm not looking for a critique):

View attachment 78417
When I tried to process the shot, I took a look at it at 100% magnification, and found something really strange:
View attachment 78418

I only saw this effect in two files on my camera, both of which were images with strong flare and heavy back-light. The camera worked perfectly well before and after these two shots.

I shot...

X-Pro2: Strange effect in a heavily back-lit shot