I'm fairly new to chickens and incubating, and most - OK, all - of what I've learned has been from books and the Internet. One of the things I've seen pushed as a hard and fast rule is that "Lockdown" means exactly that. The incubator is closed on Day 18, and is not opened after that FOR ANY REASON until ALL the chicks are hatched. You should NEVER, under ANY circumstance, open the incubator once lockdown has commenced.
So now I've had three hatches in my incubator, and I'm not sure that's always good advice. I noticed on my first hatch that once chicks start hatching, the incubator turns into a pretty wild scene, with chicks hopping around and rolling eggs about like so many little bowling balls. And today, after moving my newly hatched chicks to their new brooder, I found one egg - an expensive little Black Copper Marans - that had a perfectly formed chick in it, with a nice pip hole, with a stiff, dead beak poking through it opened up like it was gasping for air. The pip hole was face down, and the goop that had leaked from it had glued it to the floor of the incubator. Seems like with all the activity in there, the egg had been rolled over after the chick pipped, and all I could see when I looked in was an un-pipped egg.
This can't be an uncommon occurrence. I don't know if checking on my late eggs would have allowed me to catch this problem in time to save the chick, but I would like to encourage people who use the Internet to learn about incubation to also give in frequently to the urge to use common sense along with all your Internet "learning." Go with your gut. Some advice from the Internet is good. Some is bad. It can be real hard to tell which is which. But from what I've seen, when your common sense tells you to do something, do it. And whenever someone gives you a hard and fast rule, be very skeptical. Zero tolerance is for people who don't want to use their brains.
Anyway, I'd be curious to know if anyone else uses "Lockdown" as more of a suggestion than a law?