Decades ago I began using lubricated felt wads between the powder and ball in my cap and ball revolvers.
This has prevented me from seating a ball without powder.
Through the years, I've forgotten a few times to add powder before seating the lubricated wad. No big deal; the wad can be tipped sideways in the chamber with a small screwdriver, then carefully manipulated out.
I've been a proponent of using lubricated felt wads in cap and ball revolvers for decades. Not only does the wad keep the bore cleaner but it's easier to remove if you forget to add powder.
Incidentally, the earliest reference I've found to using a felt wad between ball and powder in revolvers is in a 1928 American Rifleman magazine. The practice must be older, because it was suggested to a reader who asked how to load such a revolver.
Though some people have claimed that the old timers used felt wads, I've never read or heard of a contemporary source (diary, journal, exploration notes, etc.) that verify this claim.
As long as I load with felt wads, the prospect of forgetting to add powder isn't a problem for me.