Some games have world objects which interact with the player. The characteristics of the object must be such that it makes the player believe in the world and is very important in involving the player in the game. The realism is lost in the game world if the objects don't have those characteristics.
Elder Scrolls Skyrim:
In Elder Scrolls, the horse that the player controls can easily climb the most steep mountains present in the Skyrim world. Even though the path looks impossible to climb, the horse can somehow climb and reach the peak of the tallest of mountains. This looks funny and it loses a bit of realism.
Here is the screenshot: