Last modified: 2008-09-14 12:26:10 UTC
If the "feed" parameter is anything other than "rss" or "atom", MediaWiki crashes because it tries to create an instance of an invalid class (includes/ChangesFeed.php, line 16). See URL for a live example right now in the live Wikipedia. It should instead detect that the feed format is not valid and give an HTTP 500 error.
Fixed in r40816