I have just upgraded to 2005-02-06.
In common.php in function msg() you use function headers_sent without brackets. I do not know if it works in some version of PHP, but it does not in 4.3.10. I have no idea what it does, but surely it does not what you expect, so the eventual error message is printed immediatelly, without waiting for the header to be sent.
I have noticed it at once due to "maybe another bug". I use "Remember me" when logging in. Occasionally when I return to the page I get "Sorry, username or password was wrong". Maybe bacause the cookie has expired, I'm not sure. But I guess the feedback should be better. Now the error message is printed before the headers are sent and then again after. Resulting in headers not being sent at all. Maybe these two bugs are interconnected as the cookies cannot be reset (as they are sent in headers).
I can reproduce the bug even on your site. Just now when I've opened http://wiki.splitbrain.org/wiki:dokuwiki
to report the problem, I've got "Sorry, username or password was wrong" twice. Probably because you have turned off the PHP errors, the PHP "header cannot be sent" error does not display. But when I look at the page source I see the first instance of the error message just at the beginning of the HTML code, even before <HTML> tag. The second instance of the message (the same one) is on its correct place.
The both error messages are displayed on every wiki page as long as I do not login again.
I'm using IE 6.0 on Windows 2k, if it make any difference.
Have a nice day.