Would it be because you are creating two blocks with the same name:
section that you are uncommenting
<block name="terrOreIron">
<property name="Extends" value="rConcreteCTREighth"/>
<property name="Texture" value="355"/>
<property name="DisplayInfo" value="Name"/>
<property name="ActivationDistance" value="50"/>
</block>
Second block instance further down in the file
<block name="terrOreIron">
<property name="Material" value="MoreMetal"/>
<property name="Tags" value="ore,deepOre"/>
<property name="Shape" value="Terrain"/>
<property name="Mesh" value="terrain"/>
<property name="Map.Color" value="120,70,0"/>
<property name="Texture" value="33"/>
<property name="ImposterExclude" value="true"/>
<drop event="Harvest" name="resourceScrapIron" count="105" tag="oreWoodHarvest"/>
<drop event="Harvest" name="resourceRockSmall" count="14" tag="oreWoodHarvest"/>
<drop event="Destroy" count="0"/>
<drop event="Fall" name="resourceScrapIron" count="150" prob="0.4" stick_chance="0"/>
<property name="CanMobsSpawnOn" value="true"/>
<property name="FilterTags" value="MC_outdoor,SC_terrain"/>
<property name="SortOrder1" value="d0l0"/>
<property name="SortOrder2" value="0050"/>
<property name="DisplayType" value="blockTerrain"/>
<drop event="Harvest" name="resourceRawDiamond" prob=".005" count="1" tag="artOfMiningLuckyStrikeHarvest"/>
<drop event="Harvest" name="resourceSilverNugget" prob=".01" count="1" tag="artOfMiningLuckyStrikeHarvest"/>
<drop event="Harvest" name="resourceGoldNugget" prob=".007" count="1" tag="artOfMiningLuckyStrikeHarvest"/>
<property name="DescriptionKey" value="terrainBlockGroupDesc"/>
</block>
If I was a betting man, I think it would have to do with the extends. That is old code and it extends from rConcreteCTREighth which probably doesn't exist anymore with the re-ramping of the shapes in Alpha 20.
