If the bird isn’t showing any symptoms, it may be that it once had infectious bronchitis. IB runs its course through a flock and sometimes birds will lay an occasional wrinkled egg after being infected. Something happens to the reproductive system with IB.
I'll give you a couple of links that show egg shell defects. You might look through the photos and see which you think apply. The Handbook is aimed more at commercial operations, that's why some stuff in it might not exactly apply to you, but it is still good information. You can see that there can be more than one cause for that but disease is certainly one of them.
Egg Quality Handbook
Sumi – Egg Quality
When something like this happens if it is a one-off rare occurrence I figure we are all entitled to an occasional oops. But if it becomes consistent then there is something to it. Good luck!