js/leaflet.markercluster.js
function (layer) {
    if (layer._spiderLeg) {
      this._featureGroup.removeLayer(layer);

      layer.setOpacity(1);
      //Position will be fixed up immediately in _animationUnspiderfy
      layer.setZIndexOffset(0);

      this._map.removeLayer(layer._spiderLeg);
      delete layer._spiderLeg;
    }
  }
function (layer) {
    if (layer._spiderLeg) {
      this._featureGroup.removeLayer(layer);

      layer.setOpacity(1);
      //Position will be fixed up immediately in _animationUnspiderfy
      layer.setZIndexOffset(0);

      this._map.removeLayer(layer._spiderLeg);
      delete layer._spiderLeg;
    }
  }
If the given layer is currently being spiderfied then we unspiderfy it so it isn't on the map anymore etc