Among Drupal 7's new features was a freshly rewritten Taxonomy module. Starting with that version, Drupal's venerable tool for tagging and categorizing content became a FieldAPI Field, just like Image fields, Node Reference fields, and so on. That's meant much greater flexibility for editing and displaying taxonomy terms on new Drupal sites, but it's also caused a few hiccups. In particular, when taxonomy terms are deleted (say, by an editor determined to keep things tidy), the references to them on existing nodes don't go away. Taxonomy Orphanage module is designed to fix that specific issue.
Using the module is simple -- once it's installed an enabled, the Taxonomy administration page gets an extra tab. There, you can can tell the module to hunt down and remove "dangling" references to nonexistent taxonomy terms. You can also turn on automatic scans for dangling taxonomy terms: whenever cron runs for your site, it will march through a configurable number of nodes and remove pointers to nonexistent terms as well. The use case is simple -- resolving a simple issue with Drupal core -- but Taxonomy Orphanage helps keep your site's content neat and tidy with a minium of fuss.