I had that happen, too, when I opened the lid during a hatch. I waited another 24 hours or so from the time the egg (which had been rockin' and rollin' all morning) stopped, and then I cracked it open. The chick was perfectly formed, but had died.
It's got something to do with the drop in humidity when the chicks need it most. Humidity keeps the shell soft enough for them to get thru it, I think, so when the humidity drops and the shell hardens, they never hatch. When people refer to "lockdown" on their incubators, it means literally DO NOT OPEN THE LID. DO NOT. Just don't do it. Even if one or two hatch and are in there looking bored, just keep them there. I won't make that mistake again!
I'm guessing that's what mabeyjoey is referring to ... if you open the egg and see the perfectly-formed chick that didn't make it, it means the egg got too dry.