This is a tough one, in that this term has been tortured and bended by years of use and revision. At its most basic however, affordance means something like this:
An external stimulus that reveals a function of an object.
However, different objects are perceived differently based upon who is looking at them. Also, objects can have many different interactions in real life, so common experience allows for many possible expections. Say you are playing a shooter and are armed with a crowbar. If you approach a crate in the game and "use" the crowbar, the crate may be perceived as being bashable, or simply openable depending on expectation.
Some would say it affords BOTH openability and bashability, but objects can only afford interactions that are actually possible. So, if the game only allows you to bash the crate, openability becomes a false affordance.
The problem with the term, is that people like to talk about is as being what an object "looks like it does", whereas a purist definition is forced to only look at the intersection of that set of interactions and whittle away those potential affordances which are impossible within the game world.
In short, the crate does not afford opening in with the crowbar, since that is not actually a function of the crate.
What is needed is a term to define the "forgotten children", or those perceived affordences that don't hold up in the game world.
For gaming, I'll call this an objects missed functionality. Ideally missed functionality should be for times in which an objects perception tends to lead to cognitive dissonance for the user. Example: "That wall is only waist high, I should be able to hop over it." The walls missed functionality is that of being "hop-overable", based upon the users real-life parallels (or even parallels from other games).
But we must be pragmatic here. After all, we can't predict every possible perception of our object, but we can find the common ones.
