I'll go first: When I was just starting out with chickens, I got some production dual purpose layers from Frey's. They were fine the first year, laid tons of eggs, but after their moult the second year some of them went broody. I didn't know what to expect, how long this would all take- so when I thought something should be happening under the broody hens, I went down to the coop and shook the eggs hard by my ear to see if I could tell if there was a chick in there. Those eggs didn't make it, I'd murdered the baby chicks just before they were going to hatch.

I've had three or four tries using an incubator, the first time I candled I confused the yolk with the air sac. I couldn't see anything at 3 days, I thought the egg was a dud so I broke it into the sink - and there was the little beating heart of the embryo going............ gah! The second time the average temperature was too low at the bottom of the incubator (I thought I'd be clever and use big bags of water as a heat reservoir to moderate temperature swings, but what was happening was the water was conducting the heat out through the bottom of the incubator- the temperature down there was only 96 degrees-so I tossed the eggs at around day 10 but I didn't crack them open.
I had an incukit fail each of the other times, always around day 13, where it would just shut itself off. I had the temperature plummet for I don't know how long, but I was able to successfully hatch about half of the eggs despite thinking all was lost.
I have a new-to- me incubator now, it's humming along perfectly, I'm glad it seems to be idiot proof so this time I'm hooping for success. The eggs are due in early May. :storkbaby:
So what's the worst that's happened during incubation, but you've had successful hatches?