This week's little kerfuffle over systemd and the way that its mechanism to drop superuser privileges works (see https://news.ycombinator.com/item?id=14681377 and the headlined bug report there) has led me to observe that whilst the doco for nosh, s6, daemontools, and daemontools-encore all state that their various setuidgid programs will abend on being given an unknown/invalid account, the doco for chpst in runit and runuid in perp do not come out and explicitly say the same.
runuid's doco implies it, albeit indirectly; and I have double-checked (not that I was in much doubt) that chpst actually has this behaviour. So it might be worth improving the doco for chpst and runuid to actually say this outright.