Saxon

The rule in the spec is:

The instructions in the initial template are evaluated in final output state. An instruction is evaluated in the same output state as its calling instruction, except that xsl:variable,xsl:param, xsl:with-param, xsl:attribute, xsl:comment, xsl:processing-instruction, xsl:namespace, xsl:value-of, xsl:function, xsl:key, xsl:sort, and xsl:message always evaluate the instructions in their contained sequence constructor in temporary output state.

and in looking at the code in this area I notice that Saxon doesn't seem to be setting temporary output state in all these circumstances.

Note also that the diagnostics when this rule is violated are poor; they talk of executing xsl:result-document while writing a temporary tree.

Note also that in many cases (not all) the error is detectable statically, and could result in a static warning.