Abnormal eggs are funny to talk about. As long as it is rare and not the norm every thing is fine. It is usually just a hormonal hick-up or even a grain of sand like substance.
a female chick's ovary contains all of the ova it will ever have when it's hatched. The ovary begins to convert ova to egg yolks when she is mature. With the right lighting conditions exists, hormones stimulate ova to develop into yolks. Yolks are released from the ovary into the oviduct when they reach the right size and travel down the oviduct to acquire their whites, membranes, color (if any) and shell. Sometimes things are not perfect and the shell can be squed, and even backed up and another shell layd over the top of another shell. An egg requires approximately 25 hours to complete the addition of the egg white, the shell membranes, and the shell. Soon after an egg is laid, the process starts again.