Affects Version/s: TU30/CU18/1.21
Fix Version/s: None
Affected Platforms:Xbox 360, Xbox One, PlayStation 3, PlayStation 4, PlayStation Vita
This apparently has been a bug since TU12 when new mob AI was introduced.
When one mob is pushing another mob into the wall or when there are a few mobs crowded, the mobs are pushing each other, thus going through the wall.
The mob that gets pushed inside the wall cannot attack the player but the player can kill it. And this also affects XP Orbs too.
And this bug was fixed in 1.8.1 i believe