Worst-case scenario: the UEd Goblin wipes the map and burns down your house.
From Unreal Wiki, The Unreal Engine Documentation Site
The default physics volume for areas of the level with not contained in any mapper-placed physics volume. This class should never be spawned or destroyed by UnrealScript code. Exactly one instance of this class is created at level startup to ensure the Actor.PhysicsVolume property always contains a value other than None.
 Default values
Note: At runtime the DefaultPhysicsVolume actually has
bNoDelete=True to prevent accidental destruction and
Priority=-1000000 so other PhysicsVolumes are always rated higher.
Crashes the game on purpose. The DefaultPhysicsVolume must not be destroyed during the game.