When using:
<StyledLayerDescriptor xmlns="http://www.opengis.net/sld" version="1.0.0" xsi:schemaLocation="http://www.opengis.net/sld http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><NamedLayer><Name>AAA64</Name><UserStyle><FeatureTypeStyle><Rule><Name>default</Name><TextSymbolizer><Label><ogc:PropertyName xmlns:ogc="http://www.opengis.net/ogc">ROUTE</ogc:PropertyName></Label></TextSymbolizer></Rule></FeatureTypeStyle></UserStyle></NamedLayer></StyledLayerDescriptor>
Mapserver comes back with an error saying that a specified FILTERITEM, CLASSITEM or expression key can't be found.
Leaving out the xmlns:ogc part on the node fixes the problem, but the first case should also be dealt with.