input.mss

#world {
  polygon-opacity:1;
  [zoom=1][Z_ABBREV = 6] {
    polygon-opacity:0.2;
  }
  [zoom=1][Z_NAME = 6] {
    polygon-opacity:0.3;
  }
}

output.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE Map[]>
<Map srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over" maximum-extent="-20037508.34,-20037508.34,20037508.34,20037508.34">


<Style name="world" filter-mode="first" >
  <Rule>
    <MaxScaleDenominator>500000000</MaxScaleDenominator>
    <MinScaleDenominator>200000000</MinScaleDenominator>
    <Filter>([Z_NAME] = 6)</Filter>
    <PolygonSymbolizer fill-opacity="0.3" />
  </Rule>
  <Rule>
    <Filter>([Z_NAME] = 6)</Filter>
    <PolygonSymbolizer fill-opacity="1" />
  </Rule>
  <Rule>
    <MaxScaleDenominator>500000000</MaxScaleDenominator>
    <MinScaleDenominator>200000000</MinScaleDenominator>
    <Filter>([Z_ABBREV] = 6)</Filter>
    <PolygonSymbolizer fill-opacity="0.2" />
  </Rule>
  <Rule>
    <Filter>([Z_ABBREV] = 6)</Filter>
    <PolygonSymbolizer fill-opacity="1" />
  </Rule>
  <Rule>
    <PolygonSymbolizer fill-opacity="1" />
  </Rule>
</Style>
<Layer name="world"
  srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
    <StyleName>world</StyleName>
    <Datasource>
       <Parameter name="file"><![CDATA[/Users/tmcw/src/carto/test/rendering/layers/world/931c441e-shape_demo.shp]]></Parameter>
       <Parameter name="type"><![CDATA[shape]]></Parameter>
    </Datasource>
  </Layer>

</Map>

Research:

This only affects zoom filters in the 'three rules when there should be two' case.

Current candidate is https://github.com/mapbox/carto/blob/master/lib/carto/renderer.js#L178 either calling addRules incorrect or its implementation.

Definition.prototype.addRules just dumbly adds rules to a definition.

Line 63 of filterset.js is also part of the trap.