UsernameBlacklist makes the servers crash when people try to create
accounts. This is due to the excessively complex regexes entered by
administrators. PCRE has lots of bugs which allow you to crash it with
arbitrary regexes, but as long as you keep them simple (like in
SpamBlacklist), it's not a problem.
The extension has been disabled pending a workaround.
-- Tim Starling
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