If I write a bug in deliberately is it really a bug, or just a feature that nobody asked for?

A bug is always unintentional.

If it intentional its either a feature or sabotage ;)

@Voxera <--- What she said.

Also if you agree to make a feature do something it was not designed to do - it’s a perversion.

An Easter egg

@chrisrhymes Not in my definition.

A bug is something that negatively affects the intended or requested functionality of a program.

An easter egg is a hidden extra functionality that should not break or remove any intended or requested functionality.