If the incorrect jointing method is used than it is highly likely that water will penetrate through the joint of the lead and still enter the building and/or damage the cornice itself.
If the lead is installed in oversized lengths and/or the incorrect jointing method is used, then the restriction of thermal movement will take place causing the lead to split and fail.
If the leadis clipped insufficiently than the possibility of wind lift is greatly increased with the potential for the lead to be lifted out of position and fall off the building.
Lead cornice can fail if the lead is either under or over fixed and if the incorrect fixing is used. If over fixed, the leads movement will be restricted causing it to fail. If under fixed, the potential for wind lift increases. If the wrong fixing material is used, then the lead may react with the fixing causing the fixing to fail/deteriorate overtime.