How to read from an XmlReader without moving it forwards?

Actually ReadSubtree will return a reader which "wraps" the original reader. So reading through the new one will end up advancing the original one as well. You must consider XmlReader as a forward only reader, it simply can't go back. As for your scenario, instead of trying to remember part of the XML you can ask the reader for the position in the input file. Just cast it to IXmlLineInfo interface, it has methods to return line and position. Using this you could remember some starting position (before the element in question) and then the end position of the error. And then read that part from the intput file as a plain text.

Another idea: read the outer XML (which advances the reader), then create a new reader from this XML which allows you to "go back" and process the elements of the current node.

while (r.ReadToFollowing("ParentNode"))
{
    parentXml = r.ReadOuterXml();

    //since ReadOuterXml() advances the reader to the next parent node, create a new reader to read the remaining elements of the current parent
    XmlReader r2 = XmlReader.Create(new StringReader(parentXml));
    r2.ReadToFollowing("ChildNode");
    childValue = r2.ReadElementContentAsString();
    r2.Close();
}