When running a clean wordpress install with the Twenty Eleven theme and remove the first example post and comment the following invalid happens:
There is a page telling me there are no posts available. This page contains a search bar with the same id keyword "search" as the one in the upperright corner. Two id attributes with the same keyword on one page are not allowed.
In the footer there is an element a which contains a rel attribute with the keyword "generator" which is not allowed, resulting in the following validation error:
Bad value generator for attribute rel on element a: Keyword generator is not registered.