Attached a group of invalid test cases (schema + xml document). Validator interprets all of them as valid pairs.
Example of schema:
<xsd:element name="doc">
    <xsd:complexType>
            <xsd:choice>
                <xsd:element name="elem" type="Regex" minOccurs="1" maxOccurs="unbounded"/>
            </xsd:choice>
    </xsd:complexType>
</xsd:element>

<xsd:simpleType name="Regex">
       <xsd:restriction base="xsd:string">
           <xsd:pattern value=""/>
       </xsd:restriction>
</xsd:simpleType>

The following xml document is valid for this schema:

<?xml version="1.0"?>
<doc xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance&#39;
    xsi:noNamespaceSchemaLocation='reA2.xsd' >

<!--
base='string', pattern='', value='a', type='invalid', RULE='1'
-->

     <elem>a</elem>

 </doc>