My son pointed out this odd sign at the zoo today. We all know what they meant, but if we try to parse it literally, it becomes weird. We can read it as an exhortation to employees only, not to enter. We can read it as a pair of exhortations, one not to enter, and the other that only employees should enter. On this reading, if an employee enters, she violates the first exhortation but not the second, while when a non-employee enters, she violates both exhortations, and is doing doubly wrong.
But of course what is meant is: "Do not enter, unless you are an employee." What is odd is that on this reading, the sign violates Grice's Maxim of Manner, since that point could be more briefly and less ambiguously expressed by "Employees only."