My main panel has a couple unused slots, but the metal tab covering those slots has already been removed. One of those slots has an unused breaker, the other one used to have a plastic filler panel, which eventually broke off (it's sitting on top of the panel, one of the tabs that holds it into place is broken off). So now there's an empty hole, which is obviously not very safe.
I know I could just buy a new filler plate, but for less than I'd pay for a 3-pack of filler plates, I could just fill that slot with an new 15A breaker (which I can use as a spare if I ever need it). The breaker seems like it'd be a better solution since it'll never break off like the plastic filler piece.
Is there any code restriction or other reason that would prohibit leaving unused breakers in the panel (I'll mark them as unused).
Note that the one unused breaker that's already in the panel appears to be a defunct GFCI breaker with the neutral wire clipped off, so as a secondary question -- is it ok to leave that defunct breaker in the panel?