Create a module project and insert a new window element using the wizard. Open the layer.xml. Insert a new "attr" node inside one of the generated file nodes and specify a "name" attribute with some value. Insert another attribute that is not allowed in the file node (e.g. "stuff") with some value. 
example:
<filesystem>
    <folder name="Actions">
        <folder name="Window">
            <file name="my-sandbox-TestWindowAction.instance">
                <attr name="foo" stuff="bla"/>
                <attr name="component" methodvalue="my.sandbox.TestWindowTopComponent.findInstance"/>
                <attr name="displayName" bundlevalue="my.sandbox.Bundle#CTL_TestWindowAction"/>
                <attr name="instanceCreate" methodvalue="org.openide.windows.TopComponent.openAction"/>
            </file>
        </folder>
    </folder>
</filesystem>

Validate the layer.xml

The following wrong error will be detected:
Attribute "stuff" must be declared for element type "attr".

The correct message should be e.g. Attribute "stuff" is not allowed in element type "attr".

Should be SIMPLEFIX !?