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The section on shadow attributes (attributes starting with an underscore), 3.14.2 Shadow Attributes, does not define any error conditions. The related section on Static Expressions doesn't define any either. While it seems logical to raise typical XPath expression errors, it is unclear whether the raised errors should be dynamic errors or static errors. It makes sense to make them static errors, but most expressions have defined dynamic errors that would now occur in a static context (this is also true for expressions in xsl:use-when, a section that does not define error conditions either).
There's a statement in 2.11 A dynamic error may also be signaled during the static analysis phase if the error occurs during evaluation of a static expression. But I agree it's easily missed; and the "may" is unnecessarily indecisive.
Thw WG agreed that it would be appropriate to add clarification here. I have added a paragraph to the section on static expressions to indicate that both static and dynamic errors in a static expression result in the transformation failing with a static error.
As suggested by the WG, I have also added a note concerning errors in "shadowed" attributes (where @x and @_x are both present), to the effect: The statement that the non-shadow attribute is ignored extends to error detection: it is not an error if the non-shadow attribute has an invalid value. However, this is not reflected in the schema for XSLT stylesheets, so validation using this schema may report errors in such cases.