Wall-ignoring monsters now leave their possessions on a random non-wall square next to them, and destroy walls only if no other solution is possible (I'm not sure if this will ever happen).
I can think of multiple ways how this may happen either intentionally or even by accident. At least with radius 2 ball spells (including traps, wands and alchemy) or poison (or maybe bleeding too). And maybe also with archer level 50 power or mindcraft.
I'm still not exactly sure why this new fix was introduced instead of simply restoring the old behavior, considering that monsters cannot pick up artifacts (see also this related RFE) and the ASB has been an exception for several versions now; because of this, the old behavior had no way of rendering a game unwinnable.