Could be a phpbb.com-only issue, don't know:
Got this above the page header when viewing http://www.phpbb.com/community/viewforum.php?f=26:

[phpBB Debug] PHP Notice: in file /cron.php on line 37: Undefined index: cron_lock
[phpBB Debug] PHP Notice: in file /cron.php on line 54: Undefined index: cron_lock
HTTP/1.1 200 OK Date: Wed, 25 Jul 2007 19:58:22 GMT Server: Accept-Ranges: bytes X-Powered-By: PHP/5.2.2 Cache-Control: private, no-cache="set-cookie" Expires: 0 Pragma: no-cache Keep-Alive: timeout=2, max=599 Connection: Keep-Alive Transfer-Encoding: chunked Content-Type: text/html; charset=UTF-8 1007

The second time (or the first?), http://www.phpbb.com/community/viewtopic.php?f=26&t=570347 : ... charset=UTF-8 1002