A feature which exists, but is not in the documentation. Possible reasons for this include:
- Feature came out after documentation was written
- A developer got overly fancy late one night
- The feature has not been tested at time of release deadline
- Backward or cross-vendor compatibility. Sometimes vendors want to make stuff portable from the other system, but don't wish to promote competitor's features or API's.
- Feature was never designed at all; the system behaves a certain way neither required nor forbidden by the specification, and a user finds it handy
- Bug gets good marketing