In the spirit of Jeff Atwood, circa 2007, I’ve got a golden rule of my own to claim:
If your users consider something a killer feature in your system, it should exist as a concept in your codebase.
Say you’re making an order system. The feature your users love most is merging orders together. You could let one order iterate over the contents of the other, letting a purchase order merge itself. Abiders of sghill’s Golden Rule will note it probably makes more sense to have a changeset object exist that can be applied to the purchase order.