
...which is caused by there being an edge with more than two faces conected to it. these can be very hard to find by eye, but i've been told of an easy way of doing it.
1. select your mesh.
2. go into edit mode.
3. chose either vertex or edge mode.
4. deselect all vertices/edges.
5. select --> non-manifold.
the offending part will now be highlighted.
