Hidden features
Before adding a feature into the game I like to test it for a while and get a feel for it. During that time it's available as a hidden feature and can be enabled using special commands.
This allows me to quickly test lot's of new features without cluttering the game too much. Some of these features may be added as a fully fledged feature into the main game, while others may stay hidden or be removed alltogether.
Hidden features are unstable and will have bugs.