Just because a tornado doesn't hit your house every spring or summer doesn't mean one never will. Just because a predator didn't kill one of your free ranging chickens yesterday doesn't mean one never will. I agree with Huntmaster, it is best to look at facts and then devise a strategy. Or what I consider "best practices" for me.
Whether the egg dies or not, if bacteria gets inside the egg it can multiply. Incubation temperature is about perfect for most bacteria to multiply and the egg material is the perfect food, often used by scientists to culture a bacteria. If an egg with bacteria inside it has a living embryo inside that embryo will die.
The eggshell is porous so bacteria can get inside, but the bloom forms a really good barrier as long as the bloom is intact. I've had hens hide a nest and lay a clutch for two weeks and then incubate them for three weeks without the eggs getting bacteria inside. Turkeys, ducks, and geese go longer. The bloom is really powerful but it can be washed off or scratched off. If you take a brown or green egg and rub an area with your thumb, let alone a damp cloth, you can see the color changing. That is due to pigment coming off, not the bloom. But the bloom is laid on the egg after the pigment so the bloom is long gone in that area.
If the egg has a clump of poop, dried or damp, or is really dirty with a clump of mud, dried or damp, the bloom is probably compromised. I don't set those eggs in the incubator or under a broody hen. If an egg has a real light bit of dried dirt on it I might lightly brush that off. I really try to not set dirty eggs but some are dirtier than others.
For bacteria to get inside the egg bacteria has to be present. That's why I sterilize the incubator before I set any eggs. I wipe it down with a weak bleach solution. I change the bedding under a broody hen just before I give her eggs. I do not wash the eggs, I want the bloom intact.
It is fairly rare for an egg to get bacteria in it, even if the bloom is compromised or the egg is somewhat dirty. The dirtier the egg or the more compromised the bloom the more likely it is to happen. That does not mean it is guaranteed to happen, just a bit more likely. I try to follow what I consider "best practices" and try to suggest that to others. But even if you don't it does not mean you are guaranteed problems any more than it is guaranteed you will never have a problem even if you follow them. Life doesn't work that way.
Okay, so I've heard that eggs that have not developed in in incubator and if you left them in they will stink up the incubator, and blow up?? It sounds a bit.... far fetched. Does anybody know if it's fact or fiction?
It doesn't matter if the egg is developing or not, if bacteria gets inside you have a problem. Sometimes the pressure will build up and they explode. I had one do that under a broody hen once. Often, instead of exploding they will seep a really obnoxious fluid through the porous shell. So it is possible but only if bacteria get inside.
Some people think it's best to just leave them all until day 21, but during candling I can see on day 7 that nothings in there, nothing will ever be coming out of that egg. And even if it is true.... I really don't want to clean that up!!!
I never candle eggs under a broody hen. I candle incubator eggs around Day 7 just out of curiosity. I don't remove any eggs then. I candle at 18 days when I go into lockdown and remove any that are obviously not developing. That's not because I am concerned about them exploding, just to get them out of the way while the others hatch. I see nothing wrong with removing them at Day 7 if you are confident they are not developing.