You can complete the production of a unit or building later but inside 10 turns of waiting without losing hammers. After that you lose 2% per turn. You can switch every 10 turns and not lose a single hammer for the whole game. But in practice you don't want this at all. Can be useful to keep virtual units in cities(from 1 turn of completion, like an archer) and save maintenance that way. Or save cultural buildings maintenance right before Legalism.
For wonders, you never lose production.