The 'Air-Breathing Mermaid Charm' is a useful descriptor for a certain kind of negligence in the writing of RPG books, and particularly their rules. Usually it means approximately the following phenomenon: a mermaid write-up says nothing about breathing (making it easy to assume that the setting's mermaids, being half-humanoid-half-fish, can breath both in air and water, much like mermaids in various works of fiction), but then later a Charm says that it enables air-breathing for mermaids (i.e. it solves a huge, blatant problem that wasn't made known to even exist beforehand).
I would like to know whether this is an actual race in some of the books and an actual Charm, or is it a purely apocryphal example that is very loosely based on various less catchy-sounding Charms in the game? If the former, what book(s) is the example from? If it's the latter, what canonical examples is it based on?