As shown in https://twitter.com/jhthorsen/status/455000796602855424, an internal server error occurs when you visit a page such as https://beta.musicbrainz.org/admin/user/delete/the_username_that_never_was where the user is unknown. In the particular issue from the tweet, the user most likely was impatient and clicked twice on the button - the second time, his username was already gone (replaced with "Deleted Editor #...").