listen attempts to announce each name in /rc/bin/service (for the given protocol)
which in turn causes announce to translate them using cs(8), which fails for
25_block because that's neither a port number nor
is it assigned a port number by /lib/ndb/common.
that in turn is unsurprising because it's not supposed to be given a port number;
it looks like the name resulting from mv'ing tcp25 to render it unusable
but keeping the file contents available if needed later (as opposed to simply removing it).