Paul Tonkinson's physics students have made an interesting observation. If you make a sphere of charge using the PhET charges and field simulation you find the field inside isn't zero. I tried it myself and they are right. I did this in the SMARTboard software Notebook so I could draw over the simulation. The reason for this is that this is in 2D, it's a ring not a sphere so the charges don't cancel.  To see this in 3D I used Paul Falstad's simulation.