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Usually it means that your temp got a bit high at some point in incubating. I have no doubt that mine did because we had 100 degree weather and I have no air conditioning. 
Update again I have three out and the third is hollering to the one that started yesterday so hopefully it can convince him it's time to come out of his shell lol. On top of that I have 7 more pips waiting to go still that are slowly zipping. So it looks like it will be a good hatch. Still keeping my fingers crossed though.
Lily, Please don't think I'm saying this in a smart way but I can guarantee you my bator never reached 100.    99.8 is the highest that it had gone so I don't think that is the reason.  I think what it is with mine (can't say for anyone else) is that these were shipped eggs and I sort of think that as hot as it has been they may have started to develop before putting them in the bator.  I'm sure not claiming that I know this for sure but just my thought. lol  Anyway, fantastic on your hatch. Hope they all make it.
That is always a possibility as well. My shipped eggs are hatching as well as the ones from the farm but I know I had a few temp spikes due to no AC for 2 of the batches today is day 20 for the other it is day 19.
Batch 1 3 in lock down none pipped yet. Shipped eggs Day 20
Batch 2 10 in lock down 4 hatched completely 3 externally pipped non shipped eggs day 20
Batch 3 13 in lock down 5 pipped externally none hatched yet shipped eggs day 19. 
My last hatch I got none hatching until day 21 these are hatching way early so I am thinking it's the temp. Not sure what else it could be. I want to leave the chicks in the incubator for 24 hours but gosh I am having such a hard time keeping them from destroying the incubator in the process. They are bouncing around knocking over eggs and they are not even dry yet. My special needs chick that wasn't hatched is now almost completely out of the shell but it is still stuck on his butt. Hopefully he can get it the rest of the way off.
Third to hatch 
		
		
	
	
Terrible picture but this is the second born and the special needs chick in his shell. He finally got out of it tonight now we'll see if he can learn to walk.