At this point any collapsing wormnet portals are disabled, preventing travel through them. This includes any portals that are affected by portal hammers and any portals that drift into a sector occupied by a world or another portal.  A portal can collapse even if it has just stabilized on the same turn.  This can happen, for example, if a portal drifts out of a sector containing one world and into a sector containing a different world.
 
Portals associated with warpdrone devices can collapse (and later stabilize) according to the same rules that govern non-warpdrone portals, but note that such portals are immediately destroyed whenever the corresponding warpdrone device is destroyed.