Undocumented Feature

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