Have had a few FTF's over the decades, but remarkably few, whether with factory or handloaded ammo.
Once in a while a primer slips through production without any priming compound in it, or not enough to ignite the powder. (Believe it or not, priming compound is still spread by hand, at least in all the primer factories I've visited--which include some of the major American factories.) You can usually tell if this is the case if you decap the case and look at the primer with a magnifiying glass. (Walt Berger taught me that one a while back.) The anvil can also be faulty. But the vast majority of FTF's are due to excess headspace, or a firing pin spring that's not right.
Now to address something that used to be true, but isn't any longer, at least with all the major brands of American primers I know of. Handling primers with your fingers has zero effect, because years ago manufacturers started sealing the priming compound with a water and oil-impervious coatiing. In fact you can soak 'em in water or even oil for several days, rub off the oil, and they'll still go bang. So no, there's no danger of turning a modern primer into a "dud" by handling it with your fingers.
