I just found this out the hard way today, I have a duck setting and I thought they were supposed to hatch his weekend, so I was checking them today and none were hatched but I could smell where a rotten egg had broken, but I had candled all these eggs and took out the ones that weren't forming so the rotten one had to be on that did in incubation, any way, I saw and egg that had rotten yolk on it and it had a little busted spot it, not like a pip or a crack, but like the mother duck or the Marans hen that had been laying in her nest may have broken it, anyway I figured that may be the rotten on so I just through it out and and let the duck back on her nest, then I went and found the egg and busted it and it was not rotten, and it had a live baby ducklink in it, I felt so bad to have just killed the poor little duck. The yolk and veins were still out so it had a few more days to go. But on the bright side, at least now I can stop worring about the others and I have an idea of how far along they are incubation and should have some baby duckings before next weekend.
RIP little ducky.