easter egg
An easter egg is a message or image hidden by a developer in software. Easter eggs are activated by a special operation that is not publicly released.
An easter egg is created from a sense of play on the part of the developper, and since it results in no actual harm is considered separately from bugs or computer viruses. Oftentimes a glimpse of a humorous image or facial portrait will appear.
The name derives from the practice of hunting for painted eggs on easter.