views_plugin_query_default::ensure_table may return false instead of just a string or NULL.

API page: http://api.drupal.org/api/views/plugins!views_plugin_query_default.inc/f...

The documentation declares the following:

Return value
The alias used to refer to this specific table, or NULL if the table cannot be ensured.

In fact, if you look at the 3rd if statement, you see:

if (!array_key_exists($relationship, $this->relationships)) {
    return FALSE;
  }