In games where the area that a player's available playspace is unmarked or the limits are unexplained, this can cause the players to feel frustrated when their movement is hindered for an inexplicable reason. When the level isn't based on real life physics or terrain, the area of play should be marked in some fashion. A clear boundary marked by the back ground or assets that resemble a clear limit.
Centipede
In Centipede the player has seemingly free movement due to the all black background. However it is actually li
mited by an unknown factor even though mushrooms can spawn below this line. The player doesn't exactly know when or why it will stop.