This is common for many hens to lay these once in a while. Usually new layers will lay them or very old hens right before they retire from laying all together.
If a hen consistently lays these, and I've noticed they are often passed during the night or after a period of struggling to lay and not producing anything, it may be a sign something is going wrong in her egg factory. Often, boosting her calcium intake, deep green leafy veggies or yogurt, or oyster shell can help.