Managed vs Unmanaged solution dependencies in MS Dynamics CRM 2011

A few days back I was trying to merge a couple of solutions together and when I tried to import the second solution, I was greeted with with this error:

It turns out that dependencies need to be satisfied by components that are part of the same type of solution, i.e. managed components cannot depend on unmanaged components.

I know I had read about it somewhere, but I had completely forgotten about it.