Being the stickler that I am, and pretty much self-taught, I am tolerably convinced that my early books feature my doing the same thing with words ending in -ss, because why wouldn't I? Except that then I remember reading in some manual of style that that was wrong, that -ss takes the genitive 's: it's not the princess' poodle, it's the princess's poodle.
Which I was quite happy to buy into, because of course it was a different case and needed different case-law; so I've been religiously holding to that ever since. Until, as it happens, today: when I was copy-editing away and here was and for goodness sake and it so clearly needed an apostrophe - and it struck me suddenly that no one ever in any circumstance ever has ever said "Oh, for goodness's sake!" It is, incontrovertibly, "Oh, for goodness' sake...!"
So: is there an actual rule here, or is it just custom-and-practice, with variations? Does goodness take the bare apostrophe because it's followed by another s anyway, on the front of sake? That feels to me like it could be a rule (I am trying out variants in my head here, and I think I could say "for goodness's comfort," if it only made sense, so it may just be that concatenation of sibillants that creates a special case), but I do not know. I am ignorant. Anyone want to enlighten me? Like Brutus, I pause for a reply...
*A question not uncommonly asked in our household**
**(and rarely answered fairly; she got twice as many turns as I did. House rule. A pestering sister's a festering blister, say I.)