I am getting an Index was outside the bounds of the array error when I
apply the following template select:

I do not get this error if I remove the /Field from above.  This feels
like a big nasty bug to me.

$Fields_MappedForUpdate is the result of a node-set creation like this:
<xsl:variable name="Fields_MappedForUpdate" select="fx:node-set($xx_fields)"/>