This is one told at the time of the LBJ presidency:

LBJ was in Africa speaking and as he made each promise, the crowd shouted, "Ug Wug"..."Ug Wug". At the next location, he was met by the same cry.

Feeling that his visit had been a great success, he headed back to the plane that was to take him home when he heard a voice say, "Be careful, Mr. President. Don't step in the ug wug."