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.
Are your chickens getting old? Sometimes it happens because they're sick or stressed. Other times it's just old chickens. Unless it starts happening regularly it's normally nothing to worry about.
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.