This sign was located in the bathroom of a cruise ship cabin. The purpose seems to be to warn people not to throw any trash into the toilet. But if we look closely, there seems to be a sort of double negation in play here! Perhaps what we are forbidding is not throwing trash to the toilet, or that the act of forbidding the trash-throwing is what is restricted here?
This is sort of related to the previous post about forbidding the forbidding. The context-aware reading of the sign is of course that no trash should be thrown into the toilet, and the double "X"s are meant as repeated exclamation marks to emphasize the meaning. And yet I wonder if this was the best way to convey this.