INB4 - Rule 5! It’s not a calcium deficiency. This is an egg from an older bird with insufficient albumen to support the shell during its formation. There are a number of things that can cause this including dehydration, illness, and the number one cause, old age. This is a perfectly good shell, it’s just wrinkled.
This flock is under 2 years old. Could have been stress, maybe heat? It has been crazy hot for 2 weeks, triple digits. They have lots of shade and water. Plus I till up the dirt in their run often.
Yes. Weird eggs often happen in young birds and old birds but there are other causes. Heat stress can cause this as can other stresses. If it’s just one egg just once it’s probably nothing to worry about. Make sure they have clean, fresh water and food and keep an eye on your eggs. If it starts to be a regular thing you may need to dig deeper.