A self-referential sign is one that seems to have no use other than to point itself out or even warn viewers of the dangers inherent in the sign itself.
For example, the sign below exists only to warn people that they may bump their heads on the very sign itself.
This one is just as confusing. Why does this sign exist? It isn't even a signpost it's a tree. I guess it IS a signpost once you nail a sign to it. It's very confusing.
If anyone finds other examples of self-referential signs please post links to them in the comments section above so that everyone can enjoy the joyful insanity of them.