Just had a very funny experience. I made a change on the wiki and when I returned to the weblog my posts had disappeared. Every category was empty and even permanent link urls were showing no matching entries. In panic I quickly queried the database - everything was there. I switched to IE and everything worked! I quit Firefox, thinking it must have its knickers in a twist. Still the same. I hacked some code together to dump out the query variables being passed to WordPress:

$wpvarstoreset = array('m','p','posts','w', 'cat',
    'withcomments','s','search','exact', 'sentence','poststart',
    'postend','preview','debug', 'calendar','page','paged',
    'more','tb', 'pb','author','order','orderby', 'year',
    'monthnum', 'day', 'hour', 'minute', 'second', 'name',
    'category_name', 'feed', 'author_name');
for ($i=0; $i<count($wpvarstoreset); $i += 1) {
$wpvar = $wpvarstoreset[$i];
echo ( $wpvar . "=" . $$wpvar  . "<br />n");
}

This showed that the author was being set to IanDavis. Aha! I'd just made an edit on the wiki using that name! PmWiki sets a cookie called author and PHP puts cookies into the _POST global array! WordPress was reading this and adding a bogus author parameter to all its page queries. How to fix it though? I could hack WordPress to ignore the author parameter, or I could hack PmWiki to change its cookie name. I chose the latter since, although this is a single author weblog, I might want to use WordPress and PmWiki together on other shared weblogs. I made the following change to scripts/author.php in the PmWiki installation:

if (!isset($Author)) {
  if (isset($_POST['author'])) {
    $Author = htmlspecialchars(stripmagic($_POST['author']),ENT_QUOTES);
    setcookie('wiki-author',$Author,$AuthorCookieExpires,$AuthorCookieDir);
  } else {
    $Author = htmlspecialchars(stripmagic(@$_COOKIE['wiki-author']),ENT_QUOTES);
  }
  $Author = preg_replace('/(^[^[:alpha:]]+)|[^w- ]/','',$Author);
}

Seemed to do the trick!

Update: In the original version of this post I had inadverdently changed the name of the post variable as well as the cookie. I've fixed the code above to show the correct version.