I've always done eggtopsies. I had one horrible, horrible miscalculation in which I opened an egg that broody had kicked out of the nest and I found it on day 23, stone cold. I didn't see an internal pip and I was so sure it was a dud, I didn't even do the water test. To my great sadness, I found there was still a living chick inside. When she abandoned the entire nest to be with her chicks the next day, I had the good sense to scoop up the remaining 3 eggs and put the in the incubator. One hatched day 25, and the other two on day 26.
I'm far more cautious now before chucking an egg.