The game can be designed for the ones the developers create, while the ones gamers come up with aren't always exactly meant to be done via legitimate ways.
That said, some games, especially earlier ones, were designed for gamers to create challenges. Some RPGs, like the ones where forced fights give no experience, for example. I think that developers are realizing they can point out certain challenges is a big plus, but sometimes designed challenges don't get mentioned for being too challenging, or for some other reason that eludes me at the moment.