Although PHP 5's DOM module is included by default when you compile PHP yourself, a surprising number of Linux and Unix-y distros like to remove it.
(Reported cases include FreeBSD ports and Fedora Core 6.)
This results in MediaWiki 1.12 and 1.13-svn failing due to DOMDocument class being missing, so Preprocessor_DOM gets you nowhere.
Assuming Preprocessor_Hash is functioning correctly (passes tests, and seems ok...), we should consider using it by default and dropping the DOM-based processor.
Version: unspecified
Severity: normal