Call time pass by reference...

boost.module has a call time pass by reference (which is unnecessary) that throws an error and prevents pages from being cached in certain cases.

    // Expire all files that match up
    if ($data) {
      boost_set_base_dir_in_array(&$data);
      $counter += boost_cache_expire_router($data);
    }

The actual line number with the problem is 1684.