One of the core tenets of UX is that you've got to design like "the user is drunk." Any feature of your site has to be able to be used by someone who could be drunk - because, invariably, the user will mess it up otherwise.
The User is Drunk was making the rounds at work today. I thought that anyone else with a background in UX might get a kick out of it. So far he's completed:
However, for developing the core of the game, assume that your developer is a genius and will never supply garbage values. Don't bother sanitizing function inputs if they aren't front-end accessible. A developer should know that Garbage in begets Garbage out.
But yeah, I do like the "User is drunk" mentality.