nextfreemask does nothing for proximity mines as there are no subclasses on this chain, but does bump it's enum value over the limit with the new addition to sceneobject. so killed it.

Engine/source/T3D/proximityMine.h

@@ -72,8 +72,7 @@ class ProximityMine: public Item
 protected:
    enum MaskBits {
       DeployedMask   = Parent::NextFreeMask,
-      ExplosionMask  = Parent::NextFreeMask << 1,
-      NextFreeMask   = Parent::NextFreeMask << 2
+      ExplosionMask  = Parent::NextFreeMask << 1
    };
 
    enum State