my subdiv modifier pulls the model out of shape I was following this tutorial Blender Character Modeling 2 of 10

and when I switch the subdiv modifier on and I move points about it just pulls the model out of shape, makes the surface react like weird scales and makes holes all over the place. What is my problem?
The issue you are having is caused by not connected topology, those faces are separated from the rest of the mesh. Let me illustrate:

This is how a basic plane looks with a Subdivision Surface modifier:



Notice that the edges are all rounded. Now if I extrude an edge, it will remain connected, while if I duplicate it then it is disconnected and I will need to remove doubles: