Are you using IE, perchance? Internet Explorer subverts some common error messages which can cause some real problems when debugging a web app.

If you are using mod_perl, fatalsToBrowser checks $r->bytes_sent to determine whether it should simply use $r->print to display the fatal error or if it gets to set the status code and set the message. Here are the relavent bits from fatalsToBrowser:

    # If bytes have already been sent, then
    # we print the message out directly.
    # Otherwise we make a custom error
    # handler to produce the doc for us.
    if ($r->bytes_sent) {
        $r->print($mess);
        $r->exit;
    } else {
        $r->status(500);
        $r->custom_response(500,$mess);
    }
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-- dug