hi all
This problem has been mentioned a few times, however there is not a lot we can do there. SL physics simulation is not perfect, and while in theory a physical object never can move trough an other non phantom object, SL will let it happen frequently anyway.
It may even happen that lag causes their scripts to not respond in time to turn of their physics while moving, making them go out of range and still pass the wall, even if the wall is not inside their range.
Its one of the reasons why we did choose to make cats teleport home when they need to sleep, so they at least return at that point to where they should.
Regards
KittyCatS