There is a problem when there are two unknown mesh currents passing through a current source, like the 2A source in the diagram below. Since the current source may have any voltage across it, there is no device equation to plug into the KVL equations. If you try to write the unknown voltage as:
However the unknown voltage couples the equations and is immediately eliminated, making the system indeterminate.
Instead, we use a broader interpretation of KVL and combine the meshes around the problematic current source. Now:
