Drupal.behaviours is undefined - divided by a common language?

Not for the first time I threw together a simple JQuery script to enhance a Drupal site and immediately hit a problem with Drupal.behaviours is undefined.

Many frustrated minutes later, the penny dropped: Drupal.behaviours is most certainly undefined. Drupal.behaviors should work just fine.

Beware the unconscious tendency for we Brits to use en_UK spelling!