To reproduce it, use the following setting in the httpd.conf

<Location /private/>
    AuthType None
    Require valid-user
</Location>

start the httpd server, and then use the browser to access the /private/. The browser will show “500 Internal Server Error”, while the server prints the following message in the error log

[Mon Mar 19 21:14:30.936513 2012] [core:error] [pid 3431:tid 140737100195584] [client 132.239.17.127:42029] AH00027: Buggy authn provider failed to set user for /private/

The message here is really very misleading and inaccurate.

---------

use gdb to trace the code, the problem is at server/request.c

    access_status = ap_run_access_checker_ex(r);
    if (access_status == OK) {
    ap_log_rerror(APLOG_MARK, APLOG_TRACE3, 0, r,
                  "request authorized without authentication by "
                  "access_checker_ex hook: %s", r->uri);
    }
    else if (access_status != DECLINED) {
        return decl_die(access_status, "check access", r);
    }
    else {
        if ((access_status = ap_run_check_user_id(r)) != OK) {
            return decl_die(access_status, "check user", r);
        }
        if (r->user == NULL) {
            /* don't let buggy authn module crash us in authz */
            ap_log_rerror(APLOG_MARK, APLOG_ERR, 0, r, APLOGNO(00028)
                          "Buggy authn provider failed to set user for %s",
                          r->uri);
            access_status = HTTP_INTERNAL_SERVER_ERROR;
            return decl_die(access_status, "check user", r);
        }
        ....,,
    }

since AuthType is set to None, the ap_auth_type is always set to be NULL (see set_authtype() function in modules/aaa/mod_authn_core.c). 
In the hooked functions like 

authenticate_basic_user (r=0x8b17f0) at mod_auth_basic.c:197 
authenticate_form_authn (r=0x8b17f0) at mod_auth_form.c:849
authenticate_no_user (r=0x8b17f0) at mod_authn_core.c:351

r->user cannot be set a value if ap_auth_type is NULL.

So the error occurs.