Charu and Manish have posted some of the interesting road signs they have seen during their recent travels. The most interesting one that I remember is not a real sign at all; I found it in a Ponnappa cartoon in the Bangalore edition of ToI a few (perhaps six?) years ago.
In the cartoon, you see two guys from behind them; one of them is driving a car. Through the windshield, you also see a 'road' sign. It reads:
Caution: Road Ahead!