Also worth noting: some of the cheap knockoff magsafe adaptors available on ebay have an earthing "pin" inside the adaptor (the mushroom thing) which is actuall

Also worth noting: some of the cheap knockoff magsafe adaptors available on ebay have an earthing "pin" inside the adaptor (the mushroom thing) which is actually just coated in a metallic coloured covering/paint. This doesn't actually provide contact to the existing ground pin, so you have this issue.