The documented syntax for invoking a base-class method from a PS custom class is
([<base-class>] $this).<method>(), but that doesn't seem to work, as discovered in this SO question.

Instead, the derived class' method itself is invoked, resulting in infinite recursion.

Update: The problem appears to be specific to [System.Xml.XmlNode]-derived classes; see @daxian-dbw's comment below.

A reflection-based workaround can be found in this answer.

Steps to reproduce

class child : System.Xml.XmlDocument {
  [void] LoadXml([string]$content) {
    Write-Host 'Inside overridden LoadXml.'
    # Try to call the base type's .LoadXml() method.
    ([System.Xml.XmlDocument] $this).LoadXml($content)
  }
}

[child]::new().LoadXml('<foo>bar</foo>')

Expected behavior

Inside overridden LoadXml.

The message should print once, and the XML string should be loaded into the instance via the call to the base-class method.

Actual behavior

Inside overridden LoadXml.
Inside overridden LoadXml.
...

The method keeps calling itself and eventually runs into a call-depth overflow.

Environment data

PowerShell Core v6.0.1 on macOS 10.13.3
PowerShell Core v6.0.1 on Ubuntu 16.04.3 LTS
PowerShell Core v6.0.1 on Microsoft Windows 10 Pro (64-bit; v10.0.15063)
Windows PowerShell v5.1.15063.674 on Microsoft Windows 10 Pro (64-bit; v10.0.15063)