Using a single neutral conductor for more than one circuit can be looked at as a way to save money on wiring. Because a GFCI receptacle looks for a balance of current flowing in a single hot conductor and a single neutral conductor, it will not operate properly when more than one hot conductor is sharing a neutral downstream from a GFCI. Current flowing in the neutral from the circuit not protected by the GFCI will cause the GFCI to see an imbalance typically associated with a ground fault and the GFCI will trip. A hazardous condition would exist in the circuit not protected by the GFCI as it may appear as though the power is off even though the hot leg is still live. In circuits wired this way, the neutral needs to be split at or before reaching the GFCI receptacle and a dedicated neutral must be run from the GFCI receptacle to all downstream receptacles protected by the GFCI.