That's happened to me before to, so now my new rule is to candle at day 7, only toss totally clear eggs. Check again before lockdown, and remove any that don't show chicks darkening the egg up.
Last time I hatched eggs, my temp was too low, and they were super late. 28 days for CHICKENS. I went to break into an egg, and decided to teach my husband how to do it if he ever needed to help a chick, so he had to be real careful. I thought the eggs were quitters and would have just busted them open just to see. I even waited until day 25 just to be safe! When husband got through the membrane, it moved, and bled. What?!? We quickly got it back in and resumed lockdown, and ended up with 2 chicks.
Worse hatch of my life! NONE of the Wheaten Penedesenca came out or even developed. From the same seller, I requested the extras be Golden Lakenvelder, got 4 of them, and 3 hatched. Same box, same shipping, 75% on the bonus eggs, nothing out of what I paid for. The late arrivals were Blue/Black/Splash Marans, 24 total eggs from those. 2 chicks, both pullets at laying age now, one black, one splash. Scored 1 boy and 2 girls from the Lakenvelders. The Marans, I ordered 12, she sent 12 extra, I was crazy excited. But I used the stupid thermometer that came with the incubator and it read wrong, too low.
Upgraded to a digital one with a hydrometer, and switched to ducks. The first duck eggs I found from "wild" domestic Pekins and GIANT domestic Mallards. Great fertility! Have all 3 ducklings that I set in the brooder now.
Ordered some Saxony eggs when I found the first egg, in case I didn't find anymore on the pond shore just laying around, and on the off chance it hatched, so it would have friends.
Most traumatic shipping incident ever! Of the 6 eggs, 3 were badly BROKEN, not just a simple crack on the corner egg, but basically smashed and leaking. One other had a broken yolk, very interesting to see when candling. I'd never seen anything like it before. The remaining 2, had dislodged airsacs. I set them anyways. Both are in lockdown, surprise surprise! New eggs to replace the broken ones will be here next week, we decided to finish the hatch to avoid a stagger.
The seller of the duck eggs, the best seller I have ever worked with. I asked her if she did anything "different" with her ducks to get that kind of viability. With how shaken up those eggs were, I honestly didn't expect the last two to do anything, let alone make it to lockdown. Turns out she adds vitamins to their feed for that purpose. The feed is centered around making excellent hatching eggs. Huh, I didn't know you could do that! When candling, the shells were as perfect as those from the "wild" with a forage based diet.