I do find that signs with grammatical mistakes annoy me, as do those which are garbled and incoherent. Signs normally only contain a few words, so it is surely worth checking them before they are printed....
I like the one about looking like you are under 18, which is unintentionally humorous. How do you prove that you look like you are under 18? You either do or you don't.
What really irritates me is the increased usage of icons or picture signs. From traffic lights to Windows XP - I can read, I do not need a stickman to cross the street.