Should the street sign read RESIDENTS PARKING ONLY or RESIDENTS' PARKING ONLY?
I'd supposed the latter, as it is parking FOR the residents (just like girls' school is a school FOR the girls), but it has been suggested to me that RESIDENTS is being used adjectivally and that this would imply no apostrophe.
But they often use an ordinary comma where an apostrophe is called for (and even not called for). A notable sign-writer-writ sign near me announces that a motor factors premises is "Open seven day,s a week"
P.S. Note my use of adjectival "motor factors".
"Does Magna Carta mean nothing to you? Did she die in vain?" "I try never to burn my bridges -- but an arsonist dogs my footsteps."