On suggesting a possible reference to the PHP bugs tracker, the auto linkification has replaced my valid HTML anchor with a link to the Moodle tracker.
The original hyper link text was "Bug #40163", the workaround was to use "40163":

<a title="Using UNC path to save out a file in PHP 5.1.6 and everything works great.  Upgraded to PHP 5.2.0 today and the UNC will not work anymore... Still unresolved in PHP 5.3.1... Try with 5.3.3, it works just fine here." href="http://bugs.php.net/bug.php?id=40163" target="_blank">Bug #40163</a>

I guess a filter is running on moodle.org: it should be reviewed to take into account this common use case.

Thanks in advance,
Matteo