at httpd/server/protocol.c around line

static int read_request_line(request_rec *r, apr_bucket_brigade *bb)
{
...
        rv = ap_rgetline(&(r->the_request), (apr_size_t)(r->server->limit_req_line + 2),
                         &len, r, strict ? AP_GETLINE_CRLF : 0, bb);

        if (rv != APR_SUCCESS) {
            r->request_time = apr_time_now();

            /* ap_rgetline returns APR_ENOSPC if it fills up the
             * buffer before finding the end-of-line.  This is only going to
             * happen if it exceeds the configured limit for a request-line.
             */
            if (APR_STATUS_IS_ENOSPC(rv)) {
                r->status = HTTP_REQUEST_URI_TOO_LARGE;
            }
            else if (APR_STATUS_IS_TIMEUP(rv)) {
                r->status = HTTP_REQUEST_TIME_OUT;
            }
            else if (APR_STATUS_IS_EINVAL(rv)) {
                r->status = HTTP_BAD_REQUEST;
            }
            r->proto_num = HTTP_VERSION(1,0);
            r->protocol  = "HTTP/1.0";
            return 0;
        }
...

However, the function ap_rgetline() can actually return error codes other than APR_ENOSPC, APR_TIMEUP, APR_EINVAL. If the input bb is NULL, it can even return APR_BADARG, and in some cases it returns APR_EGENERAL. These errors are ignored and HTTP status is not correctly set.