The accident we had was thinking the egg was rotten/dead since hen seemed to have abandoned it. Should have checked it with a light first to see inside before going to break the egg - was horrified to see a live wiggling full sized chick. Keeping it in a container with a light above and a sopping wet cloth covering the container. Membrane seems to have stuck/dried to the one part of the chick. Has begun chirping which we find encouraging.