Now you see, Mr. Moose points out an interesting scenario. Pearcetopher forgot the primer, and has a bucket full of charged cases with a bullet wedged in the case mouth. (Seated, for you nit-pickers.)
Now comes the test. Stupid, or reliable? You can, of course, just stuff a primer in the primer pocket as they sit, and I would venture that not one in a million would result in a KABOOM. Or, you can man up, pull all the bullets, dump all the powder, maybe bell the case mouths depending on cartridge, prime them all this time, charge them with powder - I'm going for re-using the powder you just emptied, but if you want to discard that and use fresh, no lost points - and then re-seat the bullets.
Me, I'm pulling the bullets and dumping the powder. Why? Mostly because just in case I get that one in a million, or if my stats are off, and it really is one in ??two?? I don't want to be explaining the KABOOM of a loaded cartridge going off in a priming tool by saying "Just stupid, I guess!"