The very first hatch I did, I made a horrible, uninformed decision, and part way through day 21, opened all the eggs that hadn't hatched (which was most...I messed up the humidity a little) from the big end. Crazily, of the 8 eggs out of 12 that hadn't yet hatched, 5 of them had living babies that ended up surviving somehow, even though I did all the work for them.
Of the remaining 3, 2 were duds, and one had died in the egg. I lost one of the five at day 6 in the brooder. Everyone else made it.
I've never done that since, b/c I now know better, and b/c I have more stable humidity, and typically have much better hatches, but I have opened eggs at the very end of day 22 a time or two since. I have had 1 chick survive when I did that that hadn't pipped at all yet. At that point, I figure it's unlikely that anymore are going to survive anyway, and I do like to get a count on what had developed to what point anyway.
I really think I just messed that first batch up b/c I opened the lid to take pics of the first and second eggs to hatch, and the rest of the babies would have been fine if not for that. Who knows...was just a weird fluke of beginners stupidity/luck, I think.