I heard a skit about this on “Prairie Home Companion” yesterday.

Problem:
This airport sign doesn’t say to readers what the writer intended to say.

Explanation:
The sign writer effectively is saying that the airport terminal does not smoke.

The sign writer wanted to say that occupants in the airport terminal may not smoke.

Although many sign readers will know what the sign writer wanted to say, the sign still is poorly written — probably for the sake of brevity and avoiding saying “You may not …” (because people don’t like to say “may” anymore).

Solution:
“You may not smoke in this terminal.”