We can have sentences which are not correct in posters and we forgive them.
I made a poster and was criticized for it.
It was (and this is the full sentence)
Including ghost hunting after hours.
I was told it should be
Including ghost hunt after hours.
Both read (sound) OK to me.
Is this actually correct or are both accepted?
Including ghost hunting after hours.
As a phrase, this is grammatically correct (although some people, of a poetic bent, might find the repeated gerund "including...hunting" less than euphonious).
Including ghost hunt after hours.
Hunt (unlike hunting) is a countable noun, and as such, takes an article when appearing in the singular: "Including a ghost hunt after hours". Without it, the phrase comes off as headlinese.
If this is just an advertisement, have you considered simply "Ghost hunting after hours!"? (Arguably, even "after hours" is superfluous: no one goes ghost-hunting in the light of day.)
Regarding your NEW QUESTION dude ..
Does a phrase “by itself,” such as an advertising headline, have to be grammatically correct?
the only meaningful answer is: it is incredibly well established, with literally millions of examples, that the headlines on advertising (whether 'billboards', 'magazine ads', etc) are -- observably -- usually (indeed "almost always"), simply sentence fragments rather than entire sentences.
So, your question "have to be" -- as in "SHOULD Saatchi & Saatchi ONLY USE full sentences (indeed, grammatically correct ones) for advert headlines...."
It's kind of a meaningless question. I mean, I'm all for completely banning outdoor advertising, as in Sao Paolo.
Observably it is incredibly common - essentially, "the norm" - that poster headlines are in fact "sentence fragments". Note too they are often very simply "labels".
(For example, the sign on a door "Exit" or "Toilet" is not a sentence - it's a label.)
Both are perfectly correct as the headlines on posters.
I think the second one sounds a little better - also it's shorter - but the person was totally wrong to tell you it is "grammatically incorrect".
