To fix this, go into Edit Mode, then press A to select all vertices, then press W and “Remove doubles”. that two vertices are positioned at the same location. One is that one or more vertices are duplicated, i.e. These show up as red lines and result in a model where two faces are not connected although they should be. Sometimes it so happens that a seemingly perfect model in Blender has unconnected edges in Pepakura.

I work with the combination of Blender and Pepakura for creating paper models.