Wow, Haven’t seen that in awhile. This is common when you have overlapping faces or vertices are to close together. The most common way to handle this is to select mesh scale by 10(S10{Enter}) Select armature and scale by 10. Then parent with auto-weights. Select mesh and clear scale(Alt+s). Select armature and clear scale. In some really dense meshes you will need to scale by 20. If this still doesn’t work then you most likely have faces inside the mesh or duplicate vertices.
