Ah -- I had just checked the entry under puer, thinking that any odd variants would be listed there. But apparently it's more common than I thought, enough so to merit an entry of its own, even.
Either way, I guess it's a good thing to mention exceptions. There must be other pairs like puer/puera, though I can't think of any other noun like that at the moment (I can only think of the adjective liber/libera/liberum).
Hm, I'm not sure about that, actually. The OLD has different entries for adulter and adultera as nouns and for adulter, a, um as an adjective — so did it start off as an adjective or a noun?
Adulter adultera is interesting because it's literally ad-ulter, but ulter has the feminine and neuter forms ultra ultrum.
Along the same lines there's dexter dextera/dextra dexterum/dextrum, and the -fer/-ger adjectives in general (stēlliger stēlligera stēlligerum). At a stretch into Old Latin you could make superus/super supera superum count --super is basically just pre-Classical though.