It is often the case that a feature will be added in one commit, but there isn't any client code that calls a particular method yet. As such, the method is marked
pragma: no cover
coverage passes and everyone continues along their merry way.
Later, when client code is added that exercises that code path, it would be nice if coverage.py could complain saying something like "WARNING: Block marked 'no cover' actually covered. You may want to remove the pragma." That way, we could continually make sure that the minimum amount of code is under 'no cover' pragmas.