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The XPath error appendix defines XPST0017 as follows: It is an error (the host language environment may define this error as either a static error or a dynamic error) if the expanded QName and number of arguments in a function call do not match the name and arity of a function signature in the static context. However, in the body of the text it is described unconditionally as a static error. There's a rather convoluted history here, relating to backwards compatibility. Last week, in the resolution of bug #3608, the XSL WG resolved that a call to an unknown extension function, when running in 1.0 mode, should indeed be a dynamic error rather than a static error. However, we don't need this to be stated in XPath: instead, we can use the artifice of adding all "unknown" function names to the static context for the expression, binding them to a function that raises a run-time error. This will be a different (XSLT-defined) error code. So as far as XPath is concerned, the parenthetical remark in XPST0017: (the host language environment may define this error as either a static error or a dynamic error) can be deleted.
Michael, Thank you for this suggestion. With the approval of the working groups, the parenthetical phrase you identified has been removed from the XPath specification. Regards, Don Chamberlin (for the Query and XSLT working groups)