I use a lot of hash map in my code and uses the deprecated gnu implementations that you access with
#include<ext/hash_map>

If I include this hash_map declaration before including the sparse module:
I hit the following error:

/usr/local/include/Eigen/src/Sparse/RandomSetter.h|162| error: invalid default argument for a template template parameter

I cannot figure what I did wrong before realizing that sparse matrix module use the now compliant way of using hash_map with:

#include <tr1/unordered_map>

My solution is to switch the way hash_map are included:
by including things in that order:
First eigen sparses module
Next old and deprecated ext/hash_map includes

it works.

Manuel

P.S. for those who care, I used the old implementation because it allows to use the non-standard method insert_with_no_resize that is very usefull for embedded code.