Bathroom

You're right to catch this. Contrast:

Man/lady man's/lady's

Men/ladies men's/ladies'

that help?

Actually, I don't think I got you. If we say men's room then it should be ladies' room. Right? At the same time the apostrophe is omitted in the last case.

oh the misuse of the apostrophe is so sad, isn't it?!
i never realized that they frequently leave off the apostrophe at the end of ladies'.
certainly, it should be ladies' room.

what's worse than this, i believe, is the time i was at a restaurant and saw a section of the menu headed "hamburger's"

DEATH!
haha.

Upon further reflection, I realized that the real question begging to be answered here is not, "what usage is correct," but "why is the mistaken usage so often used?".

I think what people often have in mind when they make these signs is a phrase such as "For Ladies Only" or "This room is for ladies."

Alternatively, they could be thinking of "Men" and "Ladies" not as people possessing something, but as simple names of categories. For example, here on my desk at work, I have some items I need to sort. I could label one box "Paper Clips." Another could be "Sticky Notes." A third could be "Pens and Pencils." It would be wrong as a three-dollar bill for me to write Paper Clips', Sticky Notes', or (even worse!) Pens' and Pencils'.

So, editors of bathroom plaque copy should specify that a pair of signs should read either "Men/Ladies," or "Men's/Ladies'." Or, which is even more correct, "Gentlemen" and "Ladies."

I'm with Silvana. Death to all misuser's of apostrophe's!