I post a lot of my successful efforts and a few of my "maybe" eggs, but I have never posted a complete disaster. I had one happen today so I thought I would take some photos so you could join me in my mourning over what would have been a lovely egg.
This egg cracked after I put it in the warm oven to melt the wax off to reveal the final design. This happens sometimes when I do too much etching of the shell, either through dyeing or straight vinegar etching. The shell simply gets too thin and the pressure of wiping melted wax off is enough to poke a hole right through. And this is what happened to this lovely blue poppy egg. I started with a brown chicken eggshell, etched it down to white, dyed and waxed through yellow, green, blue, jade green and then etched it again down to white. I think there may have been one other etching with vinegar somewhere in the middle because of blotchiness on the eggshell. Just too much eggshell removed and so this happened. It would have been pretty.
One interesting thing to note is the inner shell. You can see how the pores of the eggshell allowed the blue dye inside: look for the blue polka dots. This porosity can be a problem when working with lots of layers because the dye that is inside the eggshell sometimes can be pushed out the same pores when the eggshell is warmed in your hand. The only way I know how to prevent this is to let the eggshell dry out for at least a day before working on it again.