I was so depressed when I opened egg after egg to find perfectly formed chicks that just couldn't pip. I had left town and when I got home the humidity was at 40%. I opened the incubator to add water. That probably did them in. They were due to hatch already so I thought they needed the humidity, got it up to 60-65%. I was seriously bummed because I had eggs from a lovely hen that had been killed by a coyote in the incubator.