On Special:Listusers (and unfortunately some other extensions) on some small wiki when you enter the form on /somedirectory/index.php?title=Special:Listusers but when submitted you get sent to something along the lines of /somedirectory/index.php?target=&group=.
This is a result of the fact that the form uses the get method. Because wiki not using pretty urls have the title in the query this is put into the action. But most browsers may ignore the get portions in the action if the method is get instead of post. This causes the title=Special:Listusers to be ignored for sites not using pretty urls and therefore causes the Special:Listusers page on all sites not using them to be broken and always send the person to the mainpage when they try to search for a user.
This should be fixed by adding something like this:
Xml::hidden( 'title', $self->getPrefixedURL() ) .
Right below the line:
$out = Xml::openElement( 'form', array( 'method' => 'get', 'action' => $self->getLocalUrl() ) ) .