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 no physics volume specified. It is a native, not-placable actor.
- Logs a message and raises an assertion; otherise EMPTY. In other words, a DefaultPhysicsVolume object should never be deleted during the game. Probably the only reason bStatic and bNoDelete are false is so that any code that deletes one of these is immediately caught and fixed.