The one with the dent in it came down the chute while there was still an egg in the shell gland. It set up against the egg in there already and formed with a dent. For some reason your hen released two ova/yolks too close together and she tried to put a shell on them both, but couldn't do it well for the second one. If they were closer in time- you might have gotten a 'double yolker'. In this case you got a normal egg and a dented one....